<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title>Chapter 1. General Information</title><meta name="generator" content="DocBook XSL Stylesheets V1.69.1"><link rel="start" href="index.html" title="MySQL 5.0 Reference Manual"><link rel="up" href="index.html" title="MySQL 5.0 Reference Manual"><link rel="prev" href="preface.html" title="Preface"><link rel="next" href="installing.html" title="Chapter 2. Installing MySQL"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 1. General Information</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="preface.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="installing.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="introduction"></a>Chapter 1. General Information</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="section"><a href="introduction.html#manual-info">1.1. About This Manual</a></span></dt><dt><span class="section"><a href="introduction.html#manual-conventions">1.2. Conventions Used in This Manual</a></span></dt><dt><span class="section"><a href="introduction.html#what-is-mysql-ab">1.3. Overview of MySQL AB</a></span></dt><dt><span class="section"><a href="introduction.html#what-is">1.4. Overview of the MySQL Database Management System</a></span></dt><dd><dl><dt><span class="section"><a href="introduction.html#history">1.4.1. History of MySQL</a></span></dt><dt><span class="section"><a href="introduction.html#features">1.4.2. The Main Features of MySQL</a></span></dt><dt><span class="section"><a href="introduction.html#stability">1.4.3. MySQL Stability</a></span></dt><dt><span class="section"><a href="introduction.html#table-size">1.4.4. How Big MySQL Tables Can Be</a></span></dt><dt><span class="section"><a href="introduction.html#year-2000-compliance">1.4.5. Year 2000 Compliance</a></span></dt></dl></dd><dt><span class="section"><a href="introduction.html#maxdb">1.5. Overview of the MaxDB Database Management System</a></span></dt><dd><dl><dt><span class="section"><a href="introduction.html#maxdb-overview">1.5.1. What is MaxDB?</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-history">1.5.2. History of MaxDB</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-features">1.5.3. Features of MaxDB</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-licensing">1.5.4. Licensing and Support</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-mysql-differences">1.5.5. Feature Differences Between MaxDB and MySQL</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-mysql-interoperability">1.5.6. Interoperability Features Between MaxDB and MySQL</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-links">1.5.7. MaxDB-Related Links</a></span></dt></dl></dd><dt><span class="section"><a href="introduction.html#roadmap">1.6. MySQL Development Roadmap</a></span></dt><dd><dl><dt><span class="section"><a href="introduction.html#mysql-5-0-nutshell">1.6.1. What's New in MySQL 5.0</a></span></dt></dl></dd><dt><span class="section"><a href="introduction.html#mysql-information-sources">1.7. MySQL Information Sources</a></span></dt><dd><dl><dt><span class="section"><a href="introduction.html#questions">1.7.1. MySQL Mailing Lists</a></span></dt><dt><span class="section"><a href="introduction.html#irc">1.7.2. MySQL Community Support on IRC (Internet Relay Chat)</a></span></dt><dt><span class="section"><a href="introduction.html#forums">1.7.3. MySQL Community Support at the MySQL Forums</a></span></dt></dl></dd><dt><span class="section"><a href="introduction.html#compatibility">1.8. MySQL Standards Compliance</a></span></dt><dd><dl><dt><span class="section"><a href="introduction.html#standards">1.8.1. What Standards MySQL Follows</a></span></dt><dt><span class="section"><a href="introduction.html#sql-mode">1.8.2. Selecting SQL Modes</a></span></dt><dt><span class="section"><a href="introduction.html#ansi-mode">1.8.3. Running MySQL in ANSI Mode</a></span></dt><dt><span class="section"><a href="introduction.html#extensions-to-ansi">1.8.4. MySQL Extensions to Standard SQL</a></span></dt><dt><span class="section"><a href="introduction.html#differences-from-ansi">1.8.5. MySQL Differences from Standard SQL</a></span></dt><dt><span class="section"><a href="introduction.html#constraints">1.8.6. How MySQL Deals with Constraints</a></span></dt></dl></dd></dl></div><a class="indexterm" name="id2756297"></a><a class="indexterm" name="id2756303"></a><p>
The MySQL® software delivers a very fast, multi-threaded,
multi-user, and robust SQL (Structured Query Language) database
server. MySQL Server is intended for mission-critical, heavy-load
production systems as well as for embedding into mass-deployed
software. MySQL is a registered trademark of MySQL AB.
</p><p>
The MySQL software is Dual Licensed. Users can choose to use the
MySQL software as an Open Source product under the terms of the GNU
General Public License (<a href="http://www.fsf.org/licenses/" target="_top">http://www.fsf.org/licenses/</a>)
or can purchase a standard commercial license from MySQL AB. See
<a href="http://www.mysql.com/company/legal/licensing/" target="_top">http://www.mysql.com/company/legal/licensing/</a> for
more information on our licensing policies.
</p><p>
The following list describes some sections of particular interest in
this manual:
</p><div class="itemizedlist"><ul type="disc"><li><p>
For a discussion about the capabilities of the MySQL Database
Server, see <a href="introduction.html#features" title="1.4.2. The Main Features of MySQL">Section 1.4.2, “The Main Features of MySQL”</a>.
</p></li><li><p>
For installation instructions, see <a href="installing.html" title="Chapter 2. Installing MySQL">Chapter 2, <i>Installing MySQL</i></a>.
</p></li><li><p>
For tips on porting the MySQL Database Software to new
architectures or operating systems, see
<a href="porting.html" title="Appendix E. Porting to Other Systems">Appendix E, <i>Porting to Other Systems</i></a>.
</p></li><li><p>
For information about upgrading from a Version 4.1 release, see
<a href="installing.html#upgrading-from-4-1" title="2.10.2. Upgrading from Version 4.1 to 5.0">Section 2.10.2, “Upgrading from Version 4.1 to 5.0”</a>.
</p></li><li><p>
For a tutorial introduction to the MySQL Database Server, see
<a href="tutorial.html" title="Chapter 3. Tutorial">Chapter 3, <i>Tutorial</i></a>.
</p></li><li><p>
For examples of SQL and benchmarking information, see the
benchmarking directory (<code class="filename">sql-bench</code> in the
distribution).
</p></li><li><p>
For a history of new features and bugfixes, see
<a href="news.html" title="Appendix D. MySQL Change History">Appendix D, <i>MySQL Change History</i></a>.
</p></li><li><p>
For a list of currently known bugs and misfeatures, see
<a href="problems.html#bugs" title="A.8. Known Issues in MySQL">Section A.8, “Known Issues in MySQL”</a>.
</p></li><li><p>
For future plans, see <a href="introduction.html#roadmap" title="1.6. MySQL Development Roadmap">Section 1.6, “MySQL Development Roadmap”</a>.
</p></li><li><p>
For a list of all the contributors to this project, see
<a href="credits.html" title="Appendix C. Credits">Appendix C, <i>Credits</i></a>.
</p></li></ul></div><p>
<span class="bold"><strong>Important</strong></span>:
</p><p>
Reports of errors (often called “<span class="quote">bugs</span>”), as well as
questions and comments, should be sent to
<a href="http://bugs.mysql.com" target="_top">http://bugs.mysql.com</a>. See
<a href="introduction.html#bug-reports" title="1.7.1.3. How to Report Bugs or Problems">Section 1.7.1.3, “How to Report Bugs or Problems”</a>.
</p><p>
If you have found a sensitive security bug in MySQL Server, please
let us know immediately by sending an email message to
<code class="email"><<a href="mailto:security@mysql.com">security@mysql.com</a>></code>.
</p><a class="indexterm" name="id2756500"></a><a class="indexterm" name="id2756510"></a><a class="indexterm" name="id2756519"></a><a class="indexterm" name="id2756526"></a><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="manual-info"></a>1.1. About This Manual</h2></div></div></div><a class="indexterm" name="id2756547"></a><a class="indexterm" name="id2756553"></a><p>
This is the Reference Manual for the MySQL Database System,
Version 5.0, up to Release 5.0.16. It is
not intended for use with older versions of the MySQL software due
to the many functional and other differences between MySQL
5.0 and previous versions.
If you are using a Version 4.1 release of the MySQL
software, please refer to the
<em class="citetitle">MySQL 4.1 Reference Manual</em>, which provides
coverage of the 3.22, 3.23, 4.0, and 4.1 series of MySQL software
releases. Differences between minor versions of MySQL
5.0 are noted in the present text with reference to
release numbers (5.0.<em class="replaceable"><code>x</code></em>).
</p><p>
Because this manual serves as a reference, it does not provide
general instruction on SQL or relational database concepts. It
also does not teach you how to use your operating system or
command-line interpreter.
</p><p>
The MySQL Database Software is under constant development, and the
Reference Manual is updated frequently as well. The most recent
version of the manual is available online in searchable form at
<a href="http://dev.mysql.com/doc/" target="_top">http://dev.mysql.com/doc/</a>. Other formats also are available,
including HTML, PDF, and Windows CHM versions.
</p><a class="indexterm" name="id2756618"></a><a class="indexterm" name="id2756628"></a><p>
The primary document is a set of
<a href="http://docbook.org/" target="_top">DocBook</a> XML files. The
HTML version and other formats are produced automatically using,
among other tools, the
<a href="http://docbook.sourceforge.net/release/xsl/current/doc/reference.html" target="_top">DocBook
XSL stylesheets</a>.
</p><p>
If you have any suggestions concerning additions or corrections to
this manual, please send them to the documentation team at
<code class="email"><<a href="mailto:docs@mysql.com">docs@mysql.com</a>></code>.
</p><p>
This manual was initially written by David Axmark and Michael
“<span class="quote">Monty</span>” Widenius. It is maintained by the MySQL
Documentation Team, consisting of Paul DuBois, Stefan Hinz, Mike
Hillyer, and Jon Stephens. For the many other contributors, see
<a href="credits.html" title="Appendix C. Credits">Appendix C, <i>Credits</i></a>.
</p><p>
The copyright to this manual is owned by the Swedish company MySQL
AB. MySQL® and the MySQL logo are registered trademarks of
MySQL AB. Other trademarks and registered trademarks referred to
in this manual are the property of their respective owners, and
are used for identification purposes only.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="manual-conventions"></a>1.2. Conventions Used in This Manual</h2></div></div></div><p>
This manual uses certain typographical conventions:
</p><a class="indexterm" name="id2756731"></a><a class="indexterm" name="id2756742"></a><a class="indexterm" name="id2756751"></a><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">Text in this style</code> is used for SQL
statements; database, table, and column names; C and Perl code;
and environment variables. Example: “<span class="quote">To reload the grant
tables, use the <code class="literal">FLUSH PRIVILEGES</code>
statement</span>”.
</p><p>
<strong class="userinput"><code>Text in this style</code></strong> is used to indicate
input that you type in examples.
</p></li><li><p>
<span><strong class="command">Text in this style</strong></span> is used to indicate the
names of executable programs and scripts, examples being
<span><strong class="command">mysql</strong></span> (the MySQL command line client program)
and <span><strong class="command">mysqld</strong></span> (the MySQL server executable).
</p></li><li><p>
<em class="replaceable"><code>Text in this style</code></em> is used for
variable input for which you should substitute a value of your
own choosing.
</p></li><li><p>
Filenames and directory names are written like this: “<span class="quote">The
global <code class="filename">my.cnf</code> file is located in the
<code class="filename">/etc</code> directory</span>”.
</p></li><li><p>
Character sequences are written like this: “<span class="quote">To specify a
wildcard, use the ‘<code class="literal">%</code>’
character</span>”.
</p></li><li><p>
<span class="emphasis"><em>Text in this style</em></span> is used for emphasis.
</p></li><li><p>
<span class="bold"><strong>Text in this style</strong></span> is used in
table headings and to convey especially strong emphasis.
</p></li></ul></div><p>
When commands are shown that are meant to be executed from within a
particular program, the program is indicated by a prompt shown
before the command. For example, <code class="literal">shell></code>
indicates a command that you execute from your login shell, and
<code class="literal">mysql></code> indicates a statement that you execute
from the <span><strong class="command">mysql</strong></span> client program:
</p><pre class="programlisting">shell> <strong class="userinput"><code>type a shell command here</code></strong>
mysql> <strong class="userinput"><code>type a mysql statement here</code></strong>
</pre><p>
The “<span class="quote">shell</span>” is your command interpreter. On Unix, this
is typically a program such as <span><strong class="command">sh</strong></span>,
<span><strong class="command">csh</strong></span>, or <span><strong class="command">bash</strong></span>. On Windows, the
equivalent program is <span><strong class="command">command.com</strong></span> or
<span><strong class="command">cmd.exe</strong></span>, typically run in a console window.
</p><p>
When you enter a command or statement shown in an example, do not
type the prompt shown in the example.
</p><p>
Database, table, and column names must often be substituted into
statements. To indicate that such substitution is necessary, this
manual uses <em class="replaceable"><code>db_name</code></em>,
<em class="replaceable"><code>tbl_name</code></em>, and
<em class="replaceable"><code>col_name</code></em>. For example, you might see a
statement like this:
</p><pre class="programlisting">mysql> <strong class="userinput"><code>SELECT <em class="replaceable"><code>col_name</code></em> FROM <em class="replaceable"><code>db_name</code></em>.<em class="replaceable"><code>tbl_name</code></em>;</code></strong>
</pre><p>
This means that if you were to enter a similar statement, you would
supply your own database, table, and column names, perhaps like
this:
</p><pre class="programlisting">mysql> <strong class="userinput"><code>SELECT author_name FROM biblio_db.author_list;</code></strong>
</pre><p>
SQL keywords are not case sensitive and may be written in uppercase
or lowercase. This manual uses uppercase.
</p><p>
In syntax descriptions, square brackets
(‘<code class="literal">[</code>’ and
‘<code class="literal">]</code>’) are used to indicate optional
words or clauses. For example, in the following statement,
<code class="literal">IF EXISTS</code> is optional:
</p><pre class="programlisting">DROP TABLE [IF EXISTS] <em class="replaceable"><code>tbl_name</code></em>
</pre><p>
When a syntax element consists of a number of alternatives, the
alternatives are separated by vertical bars
(‘<code class="literal">|</code>’). When one member from a set of
choices <span class="emphasis"><em>may</em></span> be chosen, the alternatives are
listed within square brackets (‘<code class="literal">[</code>’
and ‘<code class="literal">]</code>’):
</p><pre class="programlisting">TRIM([[BOTH | LEADING | TRAILING] [<em class="replaceable"><code>remstr</code></em>] FROM] <em class="replaceable"><code>str</code></em>)
</pre><p>
When one member from a set of choices <span class="emphasis"><em>must</em></span> be
chosen, the alternatives are listed within braces
(‘<code class="literal">{</code>’ and
‘<code class="literal">}</code>’):
</p><pre class="programlisting">{DESCRIBE | DESC} <em class="replaceable"><code>tbl_name</code></em> [<em class="replaceable"><code>col_name</code></em> | <em class="replaceable"><code>wild</code></em>]
</pre><p>
An ellipsis (<code class="literal">...</code>) indicates the omission of a
section of a statement, typically to provide a shorter version of
more complex syntax. For example, <code class="literal">INSERT ...
SELECT</code> is shorthand for the form of
<code class="literal">INSERT</code> statement that is followed by a
<code class="literal">SELECT</code> statement.
</p><p>
An ellipsis can also indicate that the preceding syntax element of a
statement may be repeated. In the following example, multiple
<em class="replaceable"><code>reset_option</code></em> values may be given, with
each of those after the first preceded by commas:
</p><pre class="programlisting">RESET <em class="replaceable"><code>reset_option</code></em> [,<em class="replaceable"><code>reset_option</code></em>] ...
</pre><p>
Commands for setting shell variables are shown using Bourne shell
syntax. For example, the sequence to set an environment variable and
run a command looks like this in Bourne shell syntax:
</p><pre class="programlisting">shell> <strong class="userinput"><code><em class="replaceable"><code>VARNAME</code></em>=<em class="replaceable"><code>value</code></em> <em class="replaceable"><code>some_command</code></em></code></strong>
</pre><a class="indexterm" name="id2757145"></a><a class="indexterm" name="id2757154"></a><p>
If you are using <span><strong class="command">csh</strong></span> or <span><strong class="command">tcsh</strong></span>,
you must issue commands somewhat differently. You would execute the
sequence just shown like this:
</p><pre class="programlisting">shell> <strong class="userinput"><code>setenv <em class="replaceable"><code>VARNAME</code></em> <em class="replaceable"><code>value</code></em></code></strong>
shell> <strong class="userinput"><code><em class="replaceable"><code>some_command</code></em></code></strong>
</pre></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="what-is-mysql-ab"></a>1.3. Overview of MySQL AB</h2></div></div></div><a class="indexterm" name="id2757208"></a><p>
MySQL AB is the company of the MySQL founders and main developers.
MySQL AB was originally established in Sweden by David Axmark, Allan
Larsson, and Michael “<span class="quote">Monty</span>” Widenius.
</p><p>
We are dedicated to developing the MySQL database software and
promoting it to new users. MySQL AB owns the copyright to the MySQL
source code, the MySQL logo and (registered) trademark, and this
manual. See <a href="introduction.html#what-is" title="1.4. Overview of the MySQL Database Management System">Section 1.4, “Overview of the MySQL Database Management System”</a>.
</p><p>
The MySQL core values show our dedication to MySQL and Open Source.
</p><p>
These core values direct how MySQL AB works with the MySQL server
software:
</p><div class="itemizedlist"><ul type="disc"><li><p>
To be the best and the most widely used database in the world
</p></li><li><p>
To be available and affordable by all
</p></li><li><p>
To be easy to use
</p></li><li><p>
To be continuously improved while remaining fast and safe
</p></li><li><p>
To be fun to use and improve
</p></li><li><p>
To be free from bugs
</p></li></ul></div><p>
These are the core values of the company MySQL AB and its employees:
</p><div class="itemizedlist"><ul type="disc"><li><p>
We subscribe to the Open Source philosophy and support the Open
Source community
</p></li><li><p>
We aim to be good citizens
</p></li><li><p>
We prefer partners that share our values and mindset
</p></li><li><p>
We answer email and provide support
</p></li><li><p>
We are a virtual company, networking with others
</p></li><li><p>
We work against software patents
</p></li></ul></div><p>
The MySQL Web site (<a href="http://www.mysql.com/" target="_top">http://www.mysql.com/</a>) provides
the latest information about MySQL and MySQL AB.
</p><p>
By the way, the “<span class="quote">AB</span>” part of the company name is the
acronym for the Swedish “<span class="quote">aktiebolag,</span>” or “<span class="quote">stock
company.</span>” It translates to “<span class="quote">MySQL, Inc.</span>” In
fact, MySQL, Inc. and MySQL GmbH are examples of MySQL AB
subsidiaries. They are located in the US and Germany, respectively.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="what-is"></a>1.4. Overview of the MySQL Database Management System</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="introduction.html#history">1.4.1. History of MySQL</a></span></dt><dt><span class="section"><a href="introduction.html#features">1.4.2. The Main Features of MySQL</a></span></dt><dt><span class="section"><a href="introduction.html#stability">1.4.3. MySQL Stability</a></span></dt><dt><span class="section"><a href="introduction.html#table-size">1.4.4. How Big MySQL Tables Can Be</a></span></dt><dt><span class="section"><a href="introduction.html#year-2000-compliance">1.4.5. Year 2000 Compliance</a></span></dt></dl></div><a class="indexterm" name="id2757400"></a><a class="indexterm" name="id2757412"></a><p>
MySQL, the most popular Open Source SQL database management system,
is developed, distributed, and supported by MySQL AB. MySQL AB is a
commercial company, founded by the MySQL developers. It is a second
generation Open Source company that unites Open Source values and
methodology with a successful business model.
</p><p>
The MySQL Web site (<a href="http://www.mysql.com/" target="_top">http://www.mysql.com/</a>) provides
the latest information about MySQL software and MySQL AB.
</p><div class="itemizedlist"><ul type="disc"><li><p>
MySQL is a database management system.
</p><p>
A database is a structured collection of data. It may be
anything from a simple shopping list to a picture gallery or the
vast amounts of information in a corporate network. To add,
access, and process data stored in a computer database, you need
a database management system such as MySQL Server. Since
computers are very good at handling large amounts of data,
database management systems play a central role in computing, as
standalone utilities or as parts of other applications.
</p></li><li><p>
MySQL is a relational database management system.
</p><p>
<a class="indexterm" name="id2757474"></a>
A relational database stores data in separate tables rather than
putting all the data in one big storeroom. This adds speed and
flexibility. The SQL part of “<span class="quote">MySQL</span>” stands for
“<span class="quote">Structured Query Language.</span>” SQL is the most common
standardized language used to access databases and is defined by
the ANSI/ISO SQL Standard. The SQL standard has been evolving
since 1986 and several versions exist. In this manual,
“<span class="quote">SQL-92</span>” refers to the standard released in 1992,
“<span class="quote">SQL:1999</span>” refers to the standard released in 1999,
and “<span class="quote">SQL:2003</span>” refers to the current version of the
standard. We use the phrase “<span class="quote">the SQL standard</span>” to
mean the current version of the SQL Standard at any time.
</p></li><li><p>
MySQL software is Open Source.
</p><p>
<a class="indexterm" name="id2757770"></a>
<a class="indexterm" name="id2757782"></a>
<a class="indexterm" name="id2757794"></a>
<a class="indexterm" name="id2757806"></a>
Open Source means that it is possible for anyone to use and
modify the software. Anybody can download the MySQL software
from the Internet and use it without paying anything. If you
wish, you may study the source code and change it to suit your
needs. The MySQL software uses the GPL (GNU General Public
License), <a href="http://www.fsf.org/licenses/" target="_top">http://www.fsf.org/licenses/</a>, to define
what you may and may not do with the software in different
situations. If you feel uncomfortable with the GPL or need to
embed MySQL code into a commercial application, you can buy a
commercially licensed version from us. See the MySQL Licensing
Overview for more information
(<a href="http://www.mysql.com/company/legal/licensing/" target="_top">http://www.mysql.com/company/legal/licensing/</a>).
</p></li><li><p>
The MySQL Database Server is very fast, reliable, and easy to
use.
</p><p>
If that is what you are looking for, you should give it a try.
MySQL Server also has a practical set of features developed in
close cooperation with our users. You can find a performance
comparison of MySQL Server with other database managers on our
benchmark page. See <a href="optimization.html#mysql-benchmarks" title="7.1.4. The MySQL Benchmark Suite">Section 7.1.4, “The MySQL Benchmark Suite”</a>.
</p><p>
MySQL Server was originally developed to handle large databases
much faster than existing solutions and has been successfully
used in highly demanding production environments for several
years. Although under constant development, MySQL Server today
offers a rich and useful set of functions. Its connectivity,
speed, and security make MySQL Server highly suited for
accessing databases on the Internet.
</p></li><li><p>
MySQL Server works in client/server or embedded systems.
</p><p>
The MySQL Database Software is a client/server system that
consists of a multi-threaded SQL server that supports different
backends, several different client programs and libraries,
administrative tools, and a wide range of application
programming interfaces (APIs).
</p><p>
We also provide MySQL Server as an embedded multi-threaded
library that you can link into your application to get a
smaller, faster, easier-to-manage product.
</p></li><li><p>
A large amount of contributed MySQL software is available.
</p><p>
It is very likely that your favorite application or language
supports the MySQL Database Server.
</p></li></ul></div><a class="indexterm" name="id2757909"></a><a class="indexterm" name="id2757921"></a><a class="indexterm" name="id2757933"></a><p>
The official way to pronounce “<span class="quote">MySQL</span>” is “<span class="quote">My Ess
Que Ell</span>” (not “<span class="quote">my sequel</span>”), but we don't mind if
you pronounce it as “<span class="quote">my sequel</span>” or in some other
localized way.
</p><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="history"></a>1.4.1. History of MySQL</h3></div></div></div><a class="indexterm" name="id2757971"></a><a class="indexterm" name="id2757980"></a><a class="indexterm" name="id2757988"></a><a class="indexterm" name="id2757997"></a><a class="indexterm" name="id2758010"></a><a class="indexterm" name="id2758018"></a><p>
We started out with the intention of using <code class="literal">mSQL</code>
to connect to our tables using our own fast low-level (ISAM)
routines. However, after some testing, we came to the conclusion
that <code class="literal">mSQL</code> was not fast enough or flexible
enough for our needs. This resulted in a new SQL interface to our
database but with almost the same API interface as
<code class="literal">mSQL</code>. This API was designed to allow
third-party code that was written for use with
<code class="literal">mSQL</code> to be ported easily for use with MySQL.
</p><p>
The derivation of the name MySQL is not clear. Our base directory
and a large number of our libraries and tools have had the prefix
“<span class="quote">my</span>” for well over 10 years. However, co-founder
Monty Widenius's daughter is also named My. Which of the two gave
its name to MySQL is still a mystery, even for us.
</p><p>
The name of the MySQL Dolphin (our logo) is “<span class="quote">Sakila,</span>”
which was chosen by the founders of MySQL AB from a huge list of
names suggested by users in our “<span class="quote">Name the Dolphin</span>”
contest. The winning name was submitted by Ambrose Twebaze, an
Open Source software developer from Swaziland, Africa. According
to Ambrose, the feminine name Sakila has its roots in SiSwati, the
local language of Swaziland. Sakila is also the name of a town in
Arusha, Tanzania, near Ambrose's country of origin, Uganda.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="features"></a>1.4.2. The Main Features of MySQL</h3></div></div></div><a class="indexterm" name="id2758096"></a><a class="indexterm" name="id2758105"></a><p>
The following list describes some of the important characteristics
of the MySQL Database Software. See also <a href="introduction.html#roadmap" title="1.6. MySQL Development Roadmap">Section 1.6, “MySQL Development Roadmap”</a>
for more information about current and upcoming features.
</p><div class="itemizedlist"><ul type="disc"><li><p>
Internals and Portability
</p><div class="itemizedlist"><ul type="circle"><li><p>
Written in C and C++.
</p></li><li><p>
Tested with a broad range of different compilers.
</p></li><li><p>
Works on many different platforms. See
<a href="installing.html#which-os" title="2.1.1. Operating Systems Supported by MySQL">Section 2.1.1, “Operating Systems Supported by MySQL”</a>.
</p></li><li><p>
Uses GNU Automake, Autoconf, and Libtool for portability.
</p></li><li><p>
APIs for C, C++, Eiffel, Java, Perl, PHP, Python, Ruby,
and Tcl are available. See <a href="apis.html" title="Chapter 22. APIs and Libraries">Chapter 22, <i>APIs and Libraries</i></a>.
</p></li><li><p>
Fully multi-threaded using kernel threads. It can easily
use multiple CPUs if they are available.
</p></li><li><p>
Provides transactional and non-transactional storage
engines.
</p></li><li><p>
Uses very fast B-tree disk tables
(<code class="literal">MyISAM</code>) with index compression.
</p></li><li><p>
Relatively easy to add another storage engine. This is
useful if you want to add an SQL interface to an in-house
database.
</p></li><li><p>
A very fast thread-based memory allocation system.
</p></li><li><p>
Very fast joins using an optimized one-sweep multi-join.
</p></li><li><p>
In-memory hash tables, which are used as temporary tables.
</p></li><li><p>
SQL functions are implemented using a highly optimized
class library and should be as fast as possible. Usually
there is no memory allocation at all after query
initialization.
</p></li><li><p>
The MySQL code is tested with Purify (a commercial memory
leakage detector) as well as with Valgrind, a GPL tool
(<a href="http://developer.kde.org/~sewardj/" target="_top">http://developer.kde.org/~sewardj/</a>).
</p></li><li><p>
The server is available as a separate program for use in a
client/server networked environment. It is also available
as a library that can be embedded (linked) into standalone
applications. Such applications can be used in isolation
or in environments where no network is available.
</p></li></ul></div></li><li><p>
Column Types
</p><div class="itemizedlist"><ul type="circle"><li><p>
Many column types: signed/unsigned integers 1, 2, 3, 4,
and 8 bytes long, <code class="literal">FLOAT</code>,
<code class="literal">DOUBLE</code>, <code class="literal">CHAR</code>,
<code class="literal">VARCHAR</code>, <code class="literal">TEXT</code>,
<code class="literal">BLOB</code>, <code class="literal">DATE</code>,
<code class="literal">TIME</code>, <code class="literal">DATETIME</code>,
<code class="literal">TIMESTAMP</code>, <code class="literal">YEAR</code>,
<code class="literal">SET</code>, <code class="literal">ENUM</code>, and
OpenGIS spatial types. See <a href="column-types.html" title="Chapter 11. Column Types">Chapter 11, <i>Column Types</i></a>.
</p></li><li><p>
Fixed-length and variable-length records.
</p></li></ul></div></li><li><p>
Statements and Functions
</p><div class="itemizedlist"><ul type="circle"><li><p>
Full operator and function support in the
<code class="literal">SELECT</code> and <code class="literal">WHERE</code>
clauses of queries. For example:
</p><pre class="programlisting">mysql> <strong class="userinput"><code>SELECT CONCAT(first_name, ' ', last_name)</code></strong>
-> <strong class="userinput"><code>FROM citizen</code></strong>
-> <strong class="userinput"><code>WHERE income/dependents > 10000 AND age > 30;</code></strong>
</pre></li><li><p>
Full support for SQL <code class="literal">GROUP BY</code> and
<code class="literal">ORDER BY</code> clauses. Support for group
functions (<code class="literal">COUNT()</code>,
<code class="literal">COUNT(DISTINCT ...)</code>,
<code class="literal">AVG()</code>, <code class="literal">STD()</code>,
<code class="literal">SUM()</code>, <code class="literal">MAX()</code>,
<code class="literal">MIN()</code>, and
<code class="literal">GROUP_CONCAT()</code>).
</p></li><li><p>
Support for <code class="literal">LEFT OUTER JOIN</code> and
<code class="literal">RIGHT OUTER JOIN</code> with both standard SQL
and ODBC syntax.
</p></li><li><p>
Support for aliases on tables and columns as required by
standard SQL.
</p></li><li><p>
<code class="literal">DELETE</code>, <code class="literal">INSERT</code>,
<code class="literal">REPLACE</code>, and <code class="literal">UPDATE</code>
return the number of rows that were changed (affected). It
is possible to return the number of rows matched instead
by setting a flag when connecting to the server.
</p></li><li><p>
The MySQL-specific <code class="literal">SHOW</code> command can be
used to retrieve information about databases, database
engines, tables, and indexes. The
<code class="literal">EXPLAIN</code> command can be used to
determine how the optimizer resolves a query.
</p></li><li><p>
Function names do not clash with table or column names.
For example, <code class="literal">ABS</code> is a valid column
name. The only restriction is that for a function call, no
spaces are allowed between the function name and the
‘<code class="literal">(</code>’ that follows it. See
<a href="language-structure.html#reserved-words" title="9.6. Treatment of Reserved Words in MySQL">Section 9.6, “Treatment of Reserved Words in MySQL”</a>.
</p></li><li><p>
You can mix tables from different databases in the same
query (as of MySQL 3.22).
</p></li></ul></div></li><li><p>
Security
</p><div class="itemizedlist"><ul type="circle"><li><p>
A privilege and password system that is very flexible and
secure, and that allows host-based verification. Passwords
are secure because all password traffic is encrypted when
you connect to a server.
</p></li></ul></div></li><li><p>
Scalability and Limits
</p><div class="itemizedlist"><ul type="circle"><li><p>
Handles large databases. We use MySQL Server with
databases that contain 50 million records. We also know of
users who use MySQL Server with 60,000 tables and about
5,000,000,000 rows.
</p></li><li><p>
Up to 64 indexes per table are allowed (32 before MySQL
4.1.2). Each index may consist of 1 to 16 columns or parts
of columns. The maximum index width is 1000 bytes (500
before MySQL 4.1.2). An index may use a prefix of a column
for <code class="literal">CHAR</code>, <code class="literal">VARCHAR</code>,
<code class="literal">BLOB</code>, or <code class="literal">TEXT</code> column
types.
</p></li></ul></div></li><li><p>
Connectivity
</p><div class="itemizedlist"><ul type="circle"><li><p>
Clients can connect to the MySQL server using TCP/IP
sockets on any platform. On Windows systems in the NT
family (NT, 2000, XP, or 2003), clients can connect using
named pipes. On Unix systems, clients can connect using
Unix domain socket files.
</p></li><li><p>
In MySQL versions 4.1 and higher, Windows servers also
support shared-memory connections if started with the
<code class="option">--shared-memory</code> option. Clients can
connect through shared memory by using the
<code class="option">--protocol=memory</code> option.
</p></li><li><p>
The Connector/ODBC (MyODBC) interface provides MySQL
support for client programs that use ODBC (Open Database
Connectivity) connections. For example, you can use MS
Access to connect to your MySQL server. Clients can be run
on Windows or Unix. MyODBC source is available. All ODBC
2.5 functions are supported, as are many others. See
<a href="connectors.html" title="Chapter 23. Connectors">Chapter 23, <i> Connectors</i></a>.
</p></li><li><p>
The Connector/J interface provides MySQL support for Java
client programs that use JDBC connections. Clients can be
run on Windows or Unix. Connector/J source is available.
See <a href="connectors.html" title="Chapter 23. Connectors">Chapter 23, <i> Connectors</i></a>.
</p></li></ul></div></li><li><p>
Localization
</p><div class="itemizedlist"><ul type="circle"><li><p>
The server can provide error messages to clients in many
languages. See <a href="database-administration.html#languages" title="5.10.2. Setting the Error Message Language">Section 5.10.2, “Setting the Error Message Language”</a>.
</p></li><li><p>
Full support for several different character sets,
including <code class="literal">latin1</code> (cp1252),
<code class="literal">german</code>, <code class="literal">big5</code>,
<code class="literal">ujis</code>, and more. For example, the
Scandinavian characters
‘<code class="literal">å</code>’,
‘<code class="literal">ä</code>’ and
‘<code class="literal">ö</code>’ are allowed in table
and column names. Unicode support is available as of MySQL
4.1.
</p></li><li><p>
All data is saved in the chosen character set. All
comparisons for normal string columns are
case-insensitive.
</p></li><li><p>
Sorting is done according to the chosen character set
(using Swedish collation by default). It is possible to
change this when the MySQL server is started. To see an
example of very advanced sorting, look at the Czech
sorting code. MySQL Server supports many different
character sets that can be specified at compile time and
runtime.
</p></li></ul></div></li><li><p>
Clients and Tools
</p><div class="itemizedlist"><ul type="circle"><li><p>
The MySQL server has built-in support for SQL statements
to check, optimize, and repair tables. These statements
are available from the command line through the
<span><strong class="command">mysqlcheck</strong></span> client. MySQL also includes
<span><strong class="command">myisamchk</strong></span>, a very fast command-line
utility for performing these operations on
<code class="literal">MyISAM</code> tables. See
<a href="database-administration.html" title="Chapter 5. Database Administration">Chapter 5, <i>Database Administration</i></a>.
</p></li><li><p>
All MySQL programs can be invoked with the
<code class="option">--help</code> or <code class="option">-?</code> options to
obtain online assistance.
</p></li></ul></div></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="stability"></a>1.4.3. MySQL Stability</h3></div></div></div><a class="indexterm" name="id2758855"></a><p>
This section addresses the questions, “<span class="quote"><span class="emphasis"><em>How stable
is MySQL Server?</em></span></span>” and, “<span class="quote"><span class="emphasis"><em>Can I
depend on MySQL Server in this project?</em></span></span>” We will
try to clarify these issues and answer some important questions
that concern many potential users. The information in this section
is based on data gathered from the mailing lists, which are very
active in identifying problems as well as reporting types of use.
</p><p>
The original code stems back to the early 1980s. It provides a
stable code base, and the <code class="literal">ISAM</code> table format
used by the original storage engine remains backward-compatible.
At TcX, the predecessor of MySQL AB, MySQL code has worked in
projects since mid-1996, without any problems. When the MySQL
Database Software initially was released to a wider public, our
new users quickly found some pieces of untested code. Each new
release since then has had fewer portability problems, even though
each new release has also had many new features.
</p><p>
Each release of the MySQL Server has been usable. Problems have
occurred only when users try code from the “<span class="quote">gray
zones.</span>” Naturally, new users don't know what the gray zones
are; this section therefore attempts to document those areas that
are currently known. The descriptions mostly deal with Versions
3.23 and later of MySQL Server. All known and reported bugs are
fixed in the latest version, with the exception of those listed in
the bugs section, which are design-related. See
<a href="problems.html#bugs" title="A.8. Known Issues in MySQL">Section A.8, “Known Issues in MySQL”</a>.
</p><p>
The MySQL Server design is multi-layered with independent modules.
Some of the newer modules are listed here with an indication of
how well-tested each of them is:
</p><a class="indexterm" name="id2758935"></a><div class="itemizedlist"><ul type="disc"><li><p>
Replication (Stable)
</p><p>
Large groups of servers using replication are in production
use, with good results. Work on enhanced replication features
is continuing in MySQL 5.x.
</p></li><li><p>
<code class="literal">InnoDB</code> tables (Stable)
</p><p>
The <code class="literal">InnoDB</code> transactional storage engine has
been stable since version 3.23.49. <code class="literal">InnoDB</code>
is being used in large, heavy-load production systems.
</p></li><li><p>
<code class="literal">BDB</code> tables (Stable)
</p><p>
The <code class="literal">Berkeley DB</code> code is very stable, but we
are still improving the <code class="literal">BDB</code> transactional
storage engine interface in MySQL Server.
</p></li><li><p>
Full-text searches (Stable)
</p><p>
Full-text searching is widely used. Important feature
enhancements were added in MySQL 4.0 and 4.1.
</p></li><li><p>
<code class="literal">MyODBC</code> 3.51 (Stable)
</p><p>
<code class="literal">MyODBC</code> 3.51 uses ODBC SDK 3.51 and is in
wide production use. Some issues brought up appear to be
application-related and independent of the ODBC driver or
underlying database server.
</p></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="table-size"></a>1.4.4. How Big MySQL Tables Can Be</h3></div></div></div><a class="indexterm" name="id2759056"></a><a class="indexterm" name="id2759069"></a><a class="indexterm" name="id2759078"></a><a class="indexterm" name="id2759091"></a><a class="indexterm" name="id2759103"></a><p>
MySQL 3.22 had a 4GB (4 gigabyte) limit on table size. With the
<code class="literal">MyISAM</code> storage engine in MySQL 3.23, the
maximum table size was increased to 65536 terabytes
(256<sup>7</sup> – 1 bytes). With this
larger allowed table size, the maximum effective table size for
MySQL databases is usually determined by operating system
constraints on file sizes, not by MySQL internal limits.
</p><p>
The <code class="literal">InnoDB</code> storage engine maintains
<code class="literal">InnoDB</code> tables within a tablespace that can be
created from several files. This allows a table to exceed the
maximum individual file size. The tablespace can include raw disk
partitions, which allows extremely large tables. The maximum
tablespace size is 64TB.
</p><p>
The following table lists some examples of operating system
file-size limits. This is only a rough guide and is not intended
to be definitive. For the most up-to-date information, be sure to
check the documentation specific to your operating system.
</p><div class="informaltable"><table border="1"><colgroup><col><col></colgroup><tbody><tr><td><span class="bold"><strong>Operating System</strong></span></td><td><span class="bold"><strong>File-size Limit</strong></span></td></tr><tr><td>Linux 2.2-Intel 32-bit</td><td>2GB (LFS: 4GB)</td></tr><tr><td>Linux 2.4+</td><td>(using ext3 filesystem) 4TB</td></tr><tr><td>Solaris 9/10</td><td>16TB</td></tr><tr><td>NetWare w/NSS filesystem</td><td>8TB</td></tr><tr><td>win32 w/ FAT/FAT32</td><td>2GB/4GB</td></tr><tr><td>win32 w/ NTFS</td><td>2TB (possibly larger)</td></tr><tr><td>MacOS X w/ HFS+</td><td>2TB</td></tr></tbody></table></div><p>
On Linux 2.2, you can get <code class="literal">MyISAM</code> tables larger
than 2GB in size by using the Large File Support (LFS) patch for
the ext2 filesystem. On Linux 2.4, patches also exist for ReiserFS
to get support for big files (up to 2TB). Most current Linux
distributions are based on kernel 2.4 and include all the required
LFS patches. With JFS and XFS, petabyte and larger files are
possible on Linux. However, the maximum available file size still
depends on several factors, one of them being the filesystem used
to store MySQL tables.
</p><p>
For a detailed overview about LFS in Linux, have a look at Andreas
Jaeger's <em class="citetitle">Large File Support in Linux</em> page
at <a href="http://www.suse.de/~aj/linux_lfs.html" target="_top">http://www.suse.de/~aj/linux_lfs.html</a>.
</p><p>
Windows users please note: FAT and VFAT (FAT32) are
<span class="emphasis"><em>not</em></span> considered suitable for production use
with MySQL. Use NTFS instead.
</p><p>
By default, MySQL creates <code class="literal">MyISAM</code> tables with an
internal structure that allows a maximum size of about 4GB. You
can check the maximum table size for a table with the
<code class="literal">SHOW TABLE STATUS</code> statement or with
<span><strong class="command">myisamchk -dv
<em class="replaceable"><code>tbl_name</code></em></strong></span>. See
<a href="sql-syntax.html#show" title="13.5.4. SHOW Syntax">Section 13.5.4, “<code class="literal">SHOW</code> Syntax”</a>.
</p><p>
If you need a <code class="literal">MyISAM</code> table that is larger than
4GB in size (and your operating system supports large files), the
<code class="literal">CREATE TABLE</code> statement allows
<code class="literal">AVG_ROW_LENGTH</code> and <code class="literal">MAX_ROWS</code>
options. See <a href="sql-syntax.html#create-table" title="13.1.5. CREATE TABLE Syntax">Section 13.1.5, “<code class="literal">CREATE TABLE</code> Syntax”</a>. You can also change
these options with <code class="literal">ALTER TABLE</code> after the table
has been created, to increase the table's maximum allowable size.
See <a href="sql-syntax.html#alter-table" title="13.1.2. ALTER TABLE Syntax">Section 13.1.2, “<code class="literal">ALTER TABLE</code> Syntax”</a>.
</p><p>
Other ways to work around file-size limits for
<code class="literal">MyISAM</code> tables are as follows:
</p><div class="itemizedlist"><ul type="disc"><li><p>
If your large table is read-only, you can use
<span><strong class="command">myisampack</strong></span> to compress it.
<span><strong class="command">myisampack</strong></span> usually compresses a table by at
least 50%, so you can have, in effect, much bigger tables.
<span><strong class="command">myisampack</strong></span> also can merge multiple tables
into a single table. See <a href="client-side-scripts.html#myisampack" title="8.2. myisampack — Generate Compressed, Read-Only MyISAM Tables">Section 8.2, “myisampack — Generate Compressed, Read-Only MyISAM Tables”</a>.
</p></li><li><p>
MySQL includes a <code class="literal">MERGE</code> library that allows
you to handle a collection of <code class="literal">MyISAM</code> tables
that have identical structure as a single
<code class="literal">MERGE</code> table. See
<a href="storage-engines.html#merge-storage-engine" title="14.3. The MERGE Storage Engine">Section 14.3, “The <code class="literal">MERGE</code> Storage Engine”</a>.
</p></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="year-2000-compliance"></a>1.4.5. Year 2000 Compliance</h3></div></div></div><a class="indexterm" name="id2759454"></a><a class="indexterm" name="id2759464"></a><a class="indexterm" name="id2759476"></a><p>
The MySQL Server itself has no problems with Year 2000 (Y2K)
compliance:
</p><div class="itemizedlist"><ul type="disc"><li><p>
MySQL Server uses Unix time functions that handle dates into
the year <code class="literal">2037</code> for
<code class="literal">TIMESTAMP</code> values. For
<code class="literal">DATE</code> and <code class="literal">DATETIME</code>
values, dates through the year <code class="literal">9999</code> are
accepted.
</p></li><li><p>
All MySQL date functions are implemented in one source file,
<code class="filename">sql/time.cc</code>, and are coded very carefully
to be year 2000-safe.
</p></li><li><p>
In MySQL 3.22 and later, the <code class="literal">YEAR</code> column
type can store the years <code class="literal">0</code> and
<code class="literal">1901</code> to <code class="literal">2155</code> in one byte
and display them using two or four digits. All two-digit years
are considered to be in the range <code class="literal">1970</code> to
<code class="literal">2069</code>, which means that if you store
<code class="literal">01</code> in a <code class="literal">YEAR</code> column,
MySQL Server treats it as <code class="literal">2001</code>.
</p></li></ul></div><p>
The following simple demonstration illustrates that MySQL Server
has no problems with <code class="literal">DATE</code> or
<code class="literal">DATETIME</code> values through the year 9999, and no
problems with <code class="literal">TIMESTAMP</code> values until after the
year 2030:
</p><pre class="programlisting">mysql> <strong class="userinput"><code>DROP TABLE IF EXISTS y2k;</code></strong>
Query OK, 0 rows affected (0.01 sec)
mysql> <strong class="userinput"><code>CREATE TABLE y2k (date DATE,</code></strong>
-> <strong class="userinput"><code>date_time DATETIME,</code></strong>
-> <strong class="userinput"><code>time_stamp TIMESTAMP);</code></strong>
Query OK, 0 rows affected (0.01 sec)
mysql> <strong class="userinput"><code>INSERT INTO y2k VALUES</code></strong>
-> <strong class="userinput"><code>('1998-12-31','1998-12-31 23:59:59',19981231235959),</code></strong>
-> <strong class="userinput"><code>('1999-01-01','1999-01-01 00:00:00',19990101000000),</code></strong>
-> <strong class="userinput"><code>('1999-09-09','1999-09-09 23:59:59',19990909235959),</code></strong>
-> <strong class="userinput"><code>('2000-01-01','2000-01-01 00:00:00',20000101000000),</code></strong>
-> <strong class="userinput"><code>('2000-02-28','2000-02-28 00:00:00',20000228000000),</code></strong>
-> <strong class="userinput"><code>('2000-02-29','2000-02-29 00:00:00',20000229000000),</code></strong>
-> <strong class="userinput"><code>('2000-03-01','2000-03-01 00:00:00',20000301000000),</code></strong>
-> <strong class="userinput"><code>('2000-12-31','2000-12-31 23:59:59',20001231235959),</code></strong>
-> <strong class="userinput"><code>('2001-01-01','2001-01-01 00:00:00',20010101000000),</code></strong>
-> <strong class="userinput"><code>('2004-12-31','2004-12-31 23:59:59',20041231235959),</code></strong>
-> <strong class="userinput"><code>('2005-01-01','2005-01-01 00:00:00',20050101000000),</code></strong>
-> <strong class="userinput"><code>('2030-01-01','2030-01-01 00:00:00',20300101000000),</code></strong>
-> <strong class="userinput"><code>('2040-01-01','2040-01-01 00:00:00',20400101000000),</code></strong>
-> <strong class="userinput"><code>('9999-12-31','9999-12-31 23:59:59',99991231235959);</code></strong>
Query OK, 14 rows affected (0.01 sec)
Records: 14 Duplicates: 0 Warnings: 2
mysql> <strong class="userinput"><code>SELECT * FROM y2k;</code></strong>
+------------+---------------------+----------------+
| date | date_time | time_stamp |
+------------+---------------------+----------------+
| 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 |
| 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 |
| 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 |
| 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 |
| 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 |
| 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 |
| 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 |
| 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 |
| 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 |
| 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 |
| 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 |
| 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 |
| 2040-01-01 | 2040-01-01 00:00:00 | 00000000000000 |
| 9999-12-31 | 9999-12-31 23:59:59 | 00000000000000 |
+------------+---------------------+----------------+
14 rows in set (0.00 sec)
</pre><p>
The final two <code class="literal">TIMESTAMP</code> column values are zero
because the year values (<code class="literal">2040</code>,
<code class="literal">9999</code>) exceed the <code class="literal">TIMESTAMP</code>
maximum. The <code class="literal">TIMESTAMP</code> data type, which is used
to store the current time, supports values that range from
<code class="literal">19700101000000</code> to
<code class="literal">20300101000000</code> on 32-bit machines (signed
value). On 64-bit machines, <code class="literal">TIMESTAMP</code> handles
values up to <code class="literal">2106</code> (unsigned value).
</p><p>
Although MySQL Server itself is Y2K-safe, you may run into
problems if you use it with applications that are not Y2K-safe.
For example, many old applications store or manipulate years using
two-digit values (which are ambiguous) rather than four-digit
values. This problem may be compounded by applications that use
values such as <code class="literal">00</code> or <code class="literal">99</code> as
“<span class="quote">missing</span>” value indicators. Unfortunately, these
problems may be difficult to fix because different applications
may be written by different programmers, each of whom may use a
different set of conventions and date-handling functions.
</p><p>
Thus, even though MySQL Server has no Y2K problems, <span class="emphasis"><em>it
is the application's responsibility to provide unambiguous
input</em></span>. See <a href="column-types.html#y2k-issues" title="11.3.4. Y2K Issues and Date Types">Section 11.3.4, “Y2K Issues and Date Types”</a> for MySQL
Server's rules for dealing with ambiguous date input data that
contains two-digit year values.
</p></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="maxdb"></a>1.5. Overview of the MaxDB Database Management System</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="introduction.html#maxdb-overview">1.5.1. What is MaxDB?</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-history">1.5.2. History of MaxDB</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-features">1.5.3. Features of MaxDB</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-licensing">1.5.4. Licensing and Support</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-mysql-differences">1.5.5. Feature Differences Between MaxDB and MySQL</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-mysql-interoperability">1.5.6. Interoperability Features Between MaxDB and MySQL</a></span></dt><dt><span class="section"><a href="introduction.html#maxdb-links">1.5.7. MaxDB-Related Links</a></span></dt></dl></div><p>
MaxDB is a heavy-duty enterprise database. The database management
system is SAP-certified.
</p><p>
MaxDB is the new name of a database management system formerly
called SAP DB. In 2003 SAP AG and MySQL AB joined a partnership and
re-branded the database system to MaxDB. The development of MaxDB
has continued since then as it was done before – through the SAP
developer team.
</p><p>
MySQL AB co-operates closely with the MaxDB team at SAP around
delivering improvements to the MaxDB product. Joint efforts include
development of new native drivers to enable more efficient usage of
MaxDB in the Open Source community, and improvement of various
documentation to expand the MaxDB user base. Also interoperability
features between MySQL and MaxDB database are seen as important, and
for instance the new MaxDB Synchronization Manager supports data
synchronization from MaxDB to MySQL.
</p><p>
The MaxDB database management system does not share a common
code-base with the MySQL database management system. The MaxDB and
MySQL database management systems are independent products provided
by MySQL AB.
</p><p>
MySQL AB offers a complete portfolio of Professional Services for
MaxDB.
</p><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="maxdb-overview"></a>1.5.1. What is MaxDB?</h3></div></div></div><p>
MaxDB is a ANSI SQL-92 (entry level) compliant relational database
management system (RDBMS) from SAP AG, that is delivered by MySQL
AB as well. MaxDB fulfills the needs for enterprise usage: safety,
scalability high concurrency and performance . It runs on all
major operating systems. Over the years it has proven able to run
SAP R/3 and terabytes of data in 24x7 operation.
</p><p>
The database development started in 1977 as a research project at
the Technical University of Berlin . In the early 80s it became a
database product that subsequently was owned by Nixdorf, Siemens
Nixdorf, Software AG and today – by SAP AG. Along this line it
has been named VDN, Reflex, Supra 2, DDB/4, Entire SQL-DB-Server
and ADABAS D. In 1997 SAP took over the software from software AG
and renamed it to SAP DB. Since October 2000 SAP DB sources
additionally were released as open source under the GNU General
Public License (see <a href="gpl-license.html" title="Appendix J. GNU General Public License">Appendix J, <i>GNU General Public License</i></a>).
</p><p>
In 2003 SAP AG and MySQL AB joined a partnership and re-branded
the database system to MaxDB.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="maxdb-history"></a>1.5.2. History of MaxDB</h3></div></div></div><p>
The history of MaxDB goes back to SAP DB, SAP AG's DBMS, i.e.
MaxDB is a re-branded and enhanced version of SAP DB. For many
years, MaxDB has been used for small, medium, and large
installations of the mySAP Business Suite and other demanding SQL
applications requiring an enterprise-class DBMS with regard to the
number of users, the transactional workload, and the size of the
database.
</p><p>
SAP DB was meant to provide an alternative to third-party database
systems such as Oracle, Microsoft SQL Server, and DB2 by IBM. In
October 2000, SAP AG released SAP DB under the GNU GPL license
(see <a href="gpl-license.html" title="Appendix J. GNU General Public License">Appendix J, <i>GNU General Public License</i></a>), thus making it open source
software.
</p><p>
Today, MaxDB is used in about 3,500 SAP customer installations
worldwide. Moreover, the majority of all DBMS installations on
Unix and Linux within SAP’s IT department rely on MaxDB. MaxDB
is tuned towards heavy-duty online transaction processing (OLTP)
with several thousand users and database sizes ranging from
several hundred GB to multiple TB.
</p><p>
In 2003 SAP and MySQL concluded a partnership and development
cooperation agreement. As a result, SAP's database system SAP DB
has been delivered under the name of MaxDB by MySQL since the
release of version 7.5 (November 2003).
</p><p>
Version 7.5 of MaxDB is a direct advancement of the SAP DB 7.4
code base. Therefore, the MaxDB software version 7.5 can be used
as a direct upgrade of previous SAP DB versions starting 7.2.04
and higher.
</p><p>
Now, as before, the former SAP DB development team at SAP AG is
responsible for developing and supporting MaxDB. MySQL AB
co-operates closely with the MaxDB team at SAP around delivering
improvements to the MaxDB product, see <a href="introduction.html#maxdb" title="1.5. Overview of the MaxDB Database Management System">Section 1.5, “Overview of the MaxDB Database Management System”</a>.
Both SAP AG and MySQL AB handle the sale and distribution of
MaxDB. The advancement of MaxDB and the MySQL Server leverages
synergies that benefit both product lines.
</p><p>
MaxDB is subjected to SAP AG's complete quality assurance process
before it is shipped with SAP solutions or provided as a download
from the MySQL site.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="maxdb-features"></a>1.5.3. Features of MaxDB</h3></div></div></div><p>
MaxDB is a heavy-duty, SAP-certified open source database for OLTP
and OLAP usage which offers high reliability, availability,
scalability and a very comprehensive feature set. It is targeted
for large mySAP Business Suite environments and other applications
that require maximum enterprise-level database functionality and
complements the MySQL database server.
</p><p>
MaxDB operates as a client/server product. It was developed to
meet the needs of installations in OLTP and Data
Warehouse/OLAP/Decision Support scenarios.
<span class="bold"><strong>Benefits:</strong></span>
</p><div class="itemizedlist"><ul type="disc"><li><p>
<span class="bold"><strong>Easy configuration and
administration:</strong></span> GUI-based Installation Manager
and Database Manager as single administration tools for DBMS
operations
</p></li><li><p>
<span class="bold"><strong>Around-the-clock operation, no planned
downtimes, no permanent attendance required:</strong></span>
Automatic space management, no need for reorganizations
</p></li><li><p>
<span class="bold"><strong>Elaborate backup and restore
capabilities:</strong></span> Online and incremental backups,
recovery wizard to guide you through the recovery scenario
</p></li><li><p>
<span class="bold"><strong>Supports large number of users,
database sizes in the terabytes, and demanding
workloads:</strong></span> Proven reliability, performance, and
scalability
</p></li><li><p>
<span class="bold"><strong>High availability:</strong></span> Cluster
support, standby configuration, hot standby configuration
</p></li></ul></div><p>
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="maxdb-licensing"></a>1.5.4. Licensing and Support</h3></div></div></div><p>
MaxDB can be used under the same licenses available for the other
products distributed by MySQL AB. Thus, MaxDB is available under
the GNU General Public License, and a commercial license. For more
information on licensing, see
<a href="http://www.mysql.com/company/legal/licensing/" target="_top">http://www.mysql.com/company/legal/licensing/</a>.
</p><p>
MySQL AB offers MaxDB technical support to non-SAP customers. The
MaxDB support is available on various levels (Basic, Silver and
Gold), which expand from unlimited email/web-support to 24x7 phone
support for business critical systems.
</p><p>
MySQL AB also offers Licenses and Support for MaxDB when used with
SAP Applications, like SAP NetWeaver and mySAP Business Suite.
<a href="http://www.mysql.com/company/contact/" target="_top">Please
contact</a> MySQL AB for more information on licenses and
support for your needs.
</p><p>
Consulting and training services are available. MySQL gives
classes on MaxDB in regular intervals, see
<a href="http://www.mysql.com/training/" target="_top">http://www.mysql.com/training/</a> for a list of
classes.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="maxdb-mysql-differences"></a>1.5.5. Feature Differences Between MaxDB and MySQL</h3></div></div></div><p>
MaxDB is MySQL AB's SAP-certified database. The MaxDB database
server complements the MySQL AB product portfolio. Some MaxDB
features are not available on the MySQL database management server
and vice versa.
</p><p>
The following list provides a short summary of the main
differences between MaxDB and MySQL; it is not complete.
</p><div class="itemizedlist"><ul type="disc"><li><p>
MaxDB runs as a client/server system. MySQL can run as a
client/server system or as an embedded system.
</p></li><li><p>
MaxDB might not run on all platforms supported by MySQL.
</p></li><li><p>
MaxDB uses a proprietary network protocol for client/server
communication. MySQL uses either TCP/IP (with or without SSL
encryption), sockets (under Unix-like systems), or named pipes
(under Windows NT-family systems).
</p></li><li><p>
MaxDB supports stored procedures. For MySQL, stored procedures
are implemented in version 5.0. MaxDB also supports
programming of triggers through an SQL extension, which is
scheduled for MySQL 5.1. MaxDB contains a debugger for stored
procedure languages, can cascade nested triggers, and supports
multiple triggers per action and row.
</p></li><li><p>
MaxDB is distributed with user interfaces that are text-based,
graphical, or Web-based. MySQL is distributed with text-based
user interfaces only; graphical user interface (MySQL Control
Center, MySQL Administrator) are shipped separately from the
main distributions. Web-based user interfaces for MySQL are
offered by third parties.
</p></li><li><p>
MaxDB supports a number of programming interfaces that also
are supported by MySQL. For developing with MaxDB, the MaxDB
ODBC Driver, SQL Database Connectivity (SQLDBC), JDBC Driver,
Perl and Python modules and a MaxDB PHP extension, which
provides access to the MySQL MaxDB databases using PHP, are
available. Third Party Programming Interfaces: Support for OLE
DB, ADO, DAO, RDO and .NET through ODBC. MaxDB supports
embedded SQL with C/C++.
</p></li><li><p>
MaxDB includes administrative features that MySQL does not
have: job scheduling by time, event, and alert, and sending
messages to a database administrator on alert thresholds.
</p></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="maxdb-mysql-interoperability"></a>1.5.6. Interoperability Features Between MaxDB and MySQL</h3></div></div></div><p>
MaxDB and MySQL are independent database management servers. The
interoperation of the systems is possible in a way that the
systems can exchange their data. To exchange data between MaxDB
and MySQL you can use the import and export tools of the systems
or the MaxDB Synchronization Manager. The import and export tools
can be used to transfer data in a in-frequently, manual fashion.
The MaxDB Synchronization Manager offers faster, automatic data
transfer capabilities.
</p><p>
The MaxDB Loader can be used to export data and object
definitions. The Loader can export data using MaxDB internal,
binary formats and text formats (CSV). Data exported from MaxDB in
text formats can be re-imported to MySQL using the
<span><strong class="command">mysqldump</strong></span> database backup program. In order to
import MySQL data into MaxDB you can either use
<span><strong class="command">mysqldump</strong></span> to create <code class="literal">INSERT</code>
statements or <code class="literal">SELECT ... INTO OUTFILE</code> to create
a text file (CSV). Use the MaxDB Loader to load the data files
generated by MySQL.
</p><p>
Object definitions can be exchanged between the systems using
MaxDB Loader and the MySQL tool <span><strong class="command">mysqldump</strong></span>. As
the SQL dialects of both systems differ slightly and MaxDB has
features currently not supported by MySQL like SQL constraints, we
recommend to hand-tune the definition files. The
<span><strong class="command">mysqldump</strong></span> tool offers an option
<code class="literal">--compatible-name = maxdb</code> to produce output
that is compatible to MaxDB to make porting easier.
</p><p>
As part of MaxDB 7.6, the MaxDB Synchronization Manager is
released. The Synchronization Manager supports creation of
asynchronous replication scenarios between several MaxDB
instances. However, interoperability features also are planned, so
that the Synchronization Manager supports replication to and from
a MySQL server.
</p><p>
In the first release, the Synchronization Manager supports
inserting data into MySQL. This means that initially only
replication from MaxDB to MySQL is supported. In the course of
2005, exporting of data from a MySQL server to the Synchronization
Manager will be added, thus adding support for MySQL to MaxDB
replication scenarios.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="maxdb-links"></a>1.5.7. MaxDB-Related Links</h3></div></div></div><p>
The main page for information about MaxDB is
<a href="http://www.mysql.com/products/maxdb" target="_top">http://www.mysql.com/products/maxdb</a>. The page gives
details about the features of the MaxDB database management
systems and has pointers to documentation available.
</p><p>
The MySQL Reference Manual does not contain any MaxDB
documentation but the introduction given in this section. MaxDB
has it's own documentation, called the MaxDB library. The MaxDB
library is available on
<a href="http://dev.mysql.com/doc/maxdb/index.html" target="_top">http://dev.mysql.com/doc/maxdb/index.html</a>.
</p><p>
MySQL AB runs a community mailing list on MaxDB, see
<a href="http://lists.mysql.com/maxdb" target="_top">http://lists.mysql.com/maxdb</a>. The list shows a
vivid community discussion. Many of the core developers contribute
to it. Product announcements are send to list.
</p><p>
A web forum on MaxDB is on
<a href="http://forums.mysql.com/" target="_top">http://forums.mysql.com/</a>. The forum is for
questions on MaxDB not related to SAP application.
</p></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="roadmap"></a>1.6. MySQL Development Roadmap</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="introduction.html#mysql-5-0-nutshell">1.6.1. What's New in MySQL 5.0</a></span></dt></dl></div><p>
This section provides a snapshot of the MySQL development roadmap,
including major features implemented in or planned for the various
MySQL releases. The following sections provide information for
each release series.
</p><p>
The current production release series is MySQL 5.0, which was
declared stable for production use as of Version 5.0.15, released
in October 2005. The previous production release series was MySQL
4.1, which was declared stable for production use as of Version
4.1.7, released in October 2004. “<span class="quote">Production status</span>”
means that future 5.0 and 4.1 development is limited only to
bugfixes. For the older MySQL 4.0 and 3.23 series, only critical
bugfixes are made.
</p><p>
Active MySQL development is currently taking place in the MySQL
5.0 and 5.1 release series, and new features are being added only
to the latter.
</p><p>
Before upgrading from one release series to the next, please see
the notes at <a href="installing.html#upgrade" title="2.10. Upgrading MySQL">Section 2.10, “Upgrading MySQL”</a>.
</p><p>
The most requested features and the versions in which they were
implemented or are scheduled for implementation are summarized in
the following table:
</p><div class="informaltable"><table border="1"><colgroup><col><col></colgroup><tbody><tr><td><span class="bold"><strong>Feature</strong></span></td><td><span class="bold"><strong>MySQL Series</strong></span></td></tr><tr><td>Foreign keys</td><td>3.23 (for the <code class="literal">InnoDB</code> storage engine)</td></tr><tr><td>Unions</td><td>4.0</td></tr><tr><td>Subqueries</td><td>4.1</td></tr><tr><td>R-trees</td><td>4.1 (for the <code class="literal">MyISAM</code> storage engine)</td></tr><tr><td>Stored procedures</td><td>5.0</td></tr><tr><td>Views</td><td>5.0</td></tr><tr><td>Cursors</td><td>5.0</td></tr><tr><td>XA transactions</td><td>5.0</td></tr><tr><td>Foreign keys</td><td>5.1 (implemented in 3.23 for <code class="literal">InnoDB</code>)</td></tr><tr><td>Triggers</td><td>5.0 and 5.1</td></tr><tr><td>Full outer joins</td><td>5.1</td></tr><tr><td>Constraints</td><td>5.1 (implemented in 3.23 for <code class="literal">InnoDB</code>)</td></tr><tr><td>Partitioning</td><td>5.1</td></tr><tr><td>Pluggable Storage Engine API</td><td>5.1</td></tr><tr><td>Row-Based Replication</td><td>5.1</td></tr></tbody></table></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="mysql-5-0-nutshell"></a>1.6.1. What's New in MySQL 5.0</h3></div></div></div><p>
The following features are implemented in MySQL 5.0.
</p><div class="itemizedlist"><ul type="disc"><li><p>
<span class="bold"><strong><code class="literal">BIT</code> Data
Type</strong></span>: Can be used to store numbers in binary
notation. See <a href="column-types.html#numeric-type-overview" title="11.1.1. Overview of Numeric Types">Section 11.1.1, “Overview of Numeric Types”</a>.
</p></li><li><p>
<span class="bold"><strong>Cursors</strong></span>: Elementary support
for server-side cursors. See <a href="stored-procedures.html#cursors" title="17.2.11. Cursors">Section 17.2.11, “Cursors”</a>.
</p></li><li><p>
<span class="bold"><strong>Information Schema</strong></span>: The
introduction of the <code class="literal">INFORMATION_SCHEMA</code>
database in MySQL 5.0 provided a standards-compliant means
for accessing the MySQL Server's metadata, that is, data
about the databases (schemas) on the server and the objects
which they contain. See
<a href="information-schema.html" title="Chapter 20. The INFORMATION_SCHEMA Information Database">Chapter 20, <i>The <code class="literal">INFORMATION_SCHEMA</code> Information Database</i></a>.
</p></li><li><p>
<span class="bold"><strong>Instance Manager</strong></span>: Can be
used to start and stop the MySQL Server, even from a remote
host. See <a href="database-administration.html#instance-manager" title="5.2. mysqlmanager — The MySQL Instance Manager">Section 5.2, “mysqlmanager — The MySQL Instance Manager”</a>.
</p></li><li><p>
<span class="bold"><strong>Precision Math</strong></span>: MySQL 5.0
introduced stricter criteria for acceptance or rejection of
data, and implemented a new library for fixed-point
arithmetic. These contributed to a much higher degree of
accuracy for mathematical operations and greater control
over invalid values. See <a href="precision-math.html" title="Chapter 21. Precision Math">Chapter 21, <i>Precision Math</i></a>.
</p></li><li><p>
<span class="bold"><strong>Storage Engines</strong></span>: Storage
engines added in MySQL 5.0 include
<code class="literal">ARCHIVE</code> and <code class="literal">FEDERATED</code>.
See <a href="storage-engines.html#archive-storage-engine" title="14.8. The ARCHIVE Storage Engine">Section 14.8, “The <code class="literal">ARCHIVE</code> Storage Engine”</a> and
<a href="storage-engines.html#federated-storage-engine" title="14.7. The FEDERATED Storage Engine">Section 14.7, “The <code class="literal">FEDERATED</code> Storage Engine”</a>.
</p></li><li><p>
<span class="bold"><strong>Stored Routines</strong></span>: Support
for named stored procedures and stored functions was
implemented in MySQL 5.0. See
<a href="stored-procedures.html" title="Chapter 17. Stored Procedures and Functions">Chapter 17, <i>Stored Procedures and Functions</i></a>.
</p></li><li><p>
<span class="bold"><strong>Strict Mode and Standard Error
Handling</strong></span>: MySQL 5.0 added a strict mode where by
it follows standard SQL in a number of ways in which it did
not previously. Support for standard SQLSTATE error messages
was also implemented. See <a href="database-administration.html#server-sql-mode" title="5.3.2. The Server SQL Mode">Section 5.3.2, “The Server SQL Mode”</a>.
</p></li><li><p>
<span class="bold"><strong>Triggers</strong></span>: MySQL 5.0 added
limited support for triggers. See <a href="triggers.html" title="Chapter 18. Triggers">Chapter 18, <i>Triggers</i></a>
and <a href="introduction.html#ansi-diff-triggers" title="1.8.5.4. Stored Procedures and Triggers">Section 1.8.5.4, “Stored Procedures and Triggers”</a>.
</p></li><li><p>
<span class="bold"><strong><code class="literal">VARCHAR</code> Data
Type</strong></span>: The maximum effective length of a
<code class="literal">VARCHAR</code> column was increased to 65,532
bytes, and stripping of trailing whitespace was eliminated.
See <a href="column-types.html#string-types" title="11.4. String Types">Section 11.4, “String Types”</a>.
</p></li><li><p>
<span class="bold"><strong>Views</strong></span>: MySQL 5.0 added
support for named, updateable views. See
<a href="views.html" title="Chapter 19. Views">Chapter 19, <i>Views</i></a> and
<a href="introduction.html#ansi-diff-views" title="1.8.5.6. Views">Section 1.8.5.6, “Views”</a>.
</p></li><li><p>
<span class="bold"><strong>XA Transactions</strong></span>: See
<a href="sql-syntax.html#xa" title="13.4.7. XA Transactions">Section 13.4.7, “XA Transactions”</a>.
</p></li><li><p>
<span class="bold"><strong>Performance enhancements</strong></span>: A
number of improvements were made in MySQL 5.0 to improve the
speed of certain types of queries and in the handling of
certain types. These include:
</p><div class="itemizedlist"><ul type="circle"><li><p>
MySQL 5.0 introduces a new “<span class="quote">greedy</span>”
optimizer which can greatly reduce the time required to
arrive at a query execution plan. This is particularly
noticeable where several tables are to be joined and no
good join keys can otherwise be found. Without the
greedy optimizer, the complexity of the search for an
execution plan is calculated as
<code class="literal"><em class="replaceable"><code>N</code></em>!</code>, where
<em class="replaceable"><code>N</code></em> is the number of tables to
be joined. The greedy optimizer reduces this to
<code class="literal"><em class="replaceable"><code>N</code></em>!/(<em class="replaceable"><code>D</code></em>-1)!</code>,
where <em class="replaceable"><code>D</code></em> is the depth of the
search. While the greedy optimizer does not guarantee
the best possible of all execution plans (this is
currently being worked on), it can reduce the time spent
arriving at an execution plan for a join involving a
great many tables — 30, 40, or more — by a
factor of as much as 1,000. This should eliminate most
if not all situations where users thought that the
optimizer had hung when trying to perform joins across
many tables.
</p></li><li><p>
Use of the <em class="firstterm">Index Merge</em> method to
obtain better optimization of <code class="literal">AND</code> and
<code class="literal">OR</code> relations over different keys.
(Previously, these were optimized only where both
relations in the <code class="literal">WHERE</code> clause
involved the same key.) This also applies to other
one-to-one comparison operators
(<code class="literal">></code>, <code class="literal"><</code>, so
on), including <code class="literal">=</code> and the
<code class="literal">IN</code> operator. This means that MySQL
can use multiple indexes in retrieving results for
conditions such as <code class="literal">WHERE key1 > 4 OR key2
< 7</code> and even combinations of conditions
such as <code class="literal">WHERE (key1 > 4 OR key2 < 7) AND
(key3 >= 10 OR key4 = 1)</code>. See
<a href="optimization.html#index-merge-optimization" title="7.2.6. Index Merge Optimization">Section 7.2.6, “Index Merge Optimization”</a>.
</p></li><li><p>
A new equality detector finds and optimizes
“<span class="quote">hidden</span>” equalities in joins. For example,
a <code class="literal">WHERE</code> clause such as
</p><pre class="programlisting">t1.c1=t2c2 AND t2.c2=t3.c3 AND t1.c1 < 5
</pre><p>
can be reduced to
</p><pre class="programlisting">t1.c1=t3.c3 AND t2.c2 < 5 AND t3.c3 < 5
</pre><p>
These optimizations can be applied with any combination
of <code class="literal">AND</code> and <code class="literal">OR</code>
operators. See <a href="optimization.html#nested-joins" title="7.2.10. How MySQL Optimizes Nested Joins">Section 7.2.10, “How MySQL Optimizes Nested Joins”</a> and
<a href="optimization.html#outer-join-simplification" title="7.2.11. How MySQL Simplifies Outer Joins">Section 7.2.11, “How MySQL Simplifies Outer Joins”</a>.
</p></li><li><p>
Optimization of <code class="literal">NOT IN</code> and
<code class="literal">NOT BETWEEN</code> relations, reducing or
eliminating table scans for queries making use of them
by mean of range analysis. The performance of MySQL with
regard to these relations now matches its performance
with regard to <code class="literal">IN</code> and
<code class="literal">BETWEEN</code>.
</p></li><li><p>
The <code class="literal">VARCHAR</code> data type as implemented
in MySQL 5.0 is more efficient than in previous
versions, due to the elimination of the old (and
nonstandard) removal of trailing spaces during retrival.
</p></li><li><p>
The addition of a true <code class="literal">BIT</code> column
type; this type is much more efficient for storage and
retrieval of Boolean values than the workarounds
required in MySQL in versions previous to 5.0.
</p></li><li><p>
<span class="bold"><strong>Performance Improvements in the
<code class="literal">InnoDB</code> Storage Engine</strong></span>:
</p><div class="itemizedlist"><ul type="square"><li><p>
New compact storage format which can save up to 20%
of the disk space required in previous
MySQL/<code class="literal">InnoDB</code> versions.
</p></li><li><p>
Faster recovery from a failed or aborted
<code class="literal">ALTER TABLE</code>.
</p></li><li><p>
Faster implementation of
<code class="literal">TRUNCATE</code>.
</p></li></ul></div><p>
(See <a href="storage-engines.html#innodb" title="14.2. The InnoDB Storage Engine">Section 14.2, “The <code class="literal">InnoDB</code> Storage Engine”</a>.)
</p></li><li><p>
<span class="bold"><strong>Performance Improvements in the
<code class="literal">NDBCluster</code> Storage Engine</strong></span>:
</p><div class="itemizedlist"><ul type="square"><li><p>
Faster handling of queries that use
<code class="literal">IN</code> and
<code class="literal">BETWEEN</code>.
</p></li><li><p>
<span class="bold"><strong>Condition pushdown</strong></span>:
In cases involving the comparison of an unindexed
column with a constant, this condition is
“<span class="quote">pushed down</span>” to the cluster where it
is evaluated in all partitions simultaneously,
eliminating the need to send non-matching records
over the network. This can make such queries 10 to
100 times faster than in MySQL 4.1 Cluster.
</p></li></ul></div><p>
(See <a href="ndbcluster.html" title="Chapter 15. MySQL Cluster">Chapter 15, <i>MySQL Cluster</i></a>.)
</p></li></ul></div></li></ul></div><p>
For those wishing to take a look at the bleeding edge of MySQL
development, we make our BitKeeper repository for MySQL publicly
available. See <a href="installing.html#installing-source-tree" title="2.8.3. Installing from the Development Source Tree">Section 2.8.3, “Installing from the Development Source Tree”</a>.
</p></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="mysql-information-sources"></a>1.7. MySQL Information Sources</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="introduction.html#questions">1.7.1. MySQL Mailing Lists</a></span></dt><dt><span class="section"><a href="introduction.html#irc">1.7.2. MySQL Community Support on IRC (Internet Relay Chat)</a></span></dt><dt><span class="section"><a href="introduction.html#forums">1.7.3. MySQL Community Support at the MySQL Forums</a></span></dt></dl></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="questions"></a>1.7.1. MySQL Mailing Lists</h3></div></div></div><div class="toc"><dl><dt><span class="section"><a href="introduction.html#mailing-list">1.7.1.1. The MySQL Mailing Lists</a></span></dt><dt><span class="section"><a href="introduction.html#asking-questions">1.7.1.2. Asking Questions or Reporting Bugs</a></span></dt><dt><span class="section"><a href="introduction.html#bug-reports">1.7.1.3. How to Report Bugs or Problems</a></span></dt><dt><span class="section"><a href="introduction.html#answering-questions">1.7.1.4. Guidelines for Answering Questions on the Mailing List</a></span></dt></dl></div><a class="indexterm" name="id2761429"></a><a class="indexterm" name="id2761441"></a><a class="indexterm" name="id2761454"></a><p>
This section introduces the MySQL mailing lists and provides
guidelines as to how the lists should be used. When you subscribe
to a mailing list, you receive all postings to the list as email
messages. You can also send your own questions and answers to the
list.
</p><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="mailing-list"></a>1.7.1.1. The MySQL Mailing Lists</h4></div></div></div><a class="indexterm" name="id2761482"></a><a class="indexterm" name="id2761491"></a><p>
To subscribe to or unsubscribe from any of the mailing lists
described in this section, visit
<a href="http://lists.mysql.com/" target="_top">http://lists.mysql.com/</a>. For most of them, you
can select the regular version of the list where you get
individual messages, or a digest version where you get one large
message per day.
</p><p>
Please <span class="emphasis"><em>do not</em></span> send messages about
subscribing or unsubscribing to any of the mailing lists,
because such messages are distributed automatically to thousands
of other users.
</p><p>
Your local site may have many subscribers to a MySQL mailing
list. If so, the site may have a local mailing list, so that
messages sent from <code class="literal">lists.mysql.com</code> to your
site are propagated to the local list. In such cases, please
contact your system administrator to be added to or dropped from
the local MySQL list.
</p><p>
If you wish to have traffic for a mailing list go to a separate
mailbox in your mail program, set up a filter based on the
message headers. You can use either the
<code class="literal">List-ID:</code> or <code class="literal">Delivered-To:</code>
headers to identify list messages.
</p><p>
The MySQL mailing lists are as follows:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">announce</code>
</p><p>
This list is for announcements of new versions of MySQL and
related programs. This is a low-volume list to which all
MySQL users should subscribe.
</p></li><li><p>
<code class="literal">mysql</code>
</p><p>
This is the main list for general MySQL discussion. Please
note that some topics are better discussed on the
more-specialized lists. If you post to the wrong list, you
may not get an answer.
</p></li><li><p>
<code class="literal">bugs</code>
</p><p>
This list is for people who want to stay informed about
issues reported since the last release of MySQL or who want
to be actively involved in the process of bug hunting and
fixing. See <a href="introduction.html#bug-reports" title="1.7.1.3. How to Report Bugs or Problems">Section 1.7.1.3, “How to Report Bugs or Problems”</a>.
</p></li><li><p>
<code class="literal">internals</code>
</p><p>
This list is for people who work on the MySQL code. This is
also the forum for discussions on MySQL development and for
posting patches.
</p></li><li><p>
<code class="literal">mysqldoc</code>
</p><p>
This list is for people who work on the MySQL documentation:
people from MySQL AB, translators, and other community
members.
</p></li><li><p>
<code class="literal">benchmarks</code>
</p><p>
This list is for anyone interested in performance issues.
Discussions concentrate on database performance (not limited
to MySQL), but also include broader categories such as
performance of the kernel, filesystem, disk system, and so
on.
</p></li><li><p>
<code class="literal">packagers</code>
</p><p>
This list is for discussions on packaging and distributing
MySQL. This is the forum used by distribution maintainers to
exchange ideas on packaging MySQL and on ensuring that MySQL
looks and feels as similar as possible on all supported
platforms and operating systems.
</p></li><li><p>
<code class="literal">java</code>
</p><p>
This list is for discussions about the MySQL server and
Java. It is mostly used to discuss JDBC drivers, including
MySQL Connector/J.
</p></li><li><p>
<code class="literal">win32</code>
</p><p>
This list is for all topics concerning the MySQL software on
Microsoft operating systems, such as Windows 9x, Me, NT,
2000, XP, and 2003.
</p></li><li><p>
<code class="literal">myodbc</code>
</p><p>
This list is for all topics concerning connecting to the
MySQL server with ODBC.
</p></li><li><p>
<code class="literal">gui-tools</code>
</p><p>
This list is for all topics concerning MySQL GUI tools,
including <code class="literal">MySQL Administrator</code> and the
<code class="literal">MySQL Control Center</code> graphical client.
</p></li><li><p>
<code class="literal">cluster</code>
</p><p>
This list is for discussion of MySQL Cluster.
</p></li><li><p>
<code class="literal">dotnet</code>
</p><p>
This list is for discussion of the MySQL server and the .NET
platform. Mostly related to the MySQL Connector/Net
provider.
</p></li><li><p>
<code class="literal">plusplus</code>
</p><p>
This list is for all topics concerning programming with the
C++ API for MySQL.
</p></li><li><p>
<code class="literal">perl</code>
</p><p>
This list is for all topics concerning the Perl support for
MySQL with <code class="literal">DBD::mysql</code>.
</p></li></ul></div><p>
If you're unable to get an answer to your questions from a MySQL
mailing list or forum, one option is to purchase support from
MySQL AB. This puts you in direct contact with MySQL developers.
</p><p>
The following table shows some MySQL mailing lists in languages
other than English. These lists are not operated by MySQL AB.
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal"><code class="email"><<a href="mailto:mysql-france-subscribe@yahoogroups.com">mysql-france-subscribe@yahoogroups.com</a>></code></code>
</p><p>
A French mailing list.
</p></li><li><p>
<code class="literal"><code class="email"><<a href="mailto:list@tinc.net">list@tinc.net</a>></code></code>
</p><p>
A Korean mailing list. Email <code class="literal">subscribe mysql
your@email.address</code> to this list.
</p></li><li><p>
<code class="literal"><code class="email"><<a href="mailto:mysql-de-request@lists.4t2.com">mysql-de-request@lists.4t2.com</a>></code></code>
</p><p>
A German mailing list. Email <code class="literal">subscribe mysql-de
your@email.address</code> to this list. You can find
information about this mailing list at
<a href="http://www.4t2.com/mysql/" target="_top">http://www.4t2.com/mysql/</a>.
</p></li><li><p>
<code class="literal"><code class="email"><<a href="mailto:mysql-br-request@listas.linkway.com.br">mysql-br-request@listas.linkway.com.br</a>></code></code>
</p><p>
A Portuguese mailing list. Email <code class="literal">subscribe mysql-br
your@email.address</code> to this list.
</p></li><li><p>
<code class="literal"><code class="email"><<a href="mailto:mysql-alta@elistas.net">mysql-alta@elistas.net</a>></code></code>
</p><p>
A Spanish mailing list. Email <code class="literal">subscribe mysql
your@email.address</code> to this list.
</p></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="asking-questions"></a>1.7.1.2. Asking Questions or Reporting Bugs</h4></div></div></div><a class="indexterm" name="id2761949"></a><a class="indexterm" name="id2761958"></a><a class="indexterm" name="id2761970"></a><p>
Before posting a bug report or question, please do the
following:
</p><div class="itemizedlist"><ul type="disc"><li><p>
Start by searching the MySQL online manual at
<a href="http://dev.mysql.com/doc/" target="_top">http://dev.mysql.com/doc/</a>. We try to keep the
manual up to date by updating it frequently with solutions
to newly found problems. The change history
(<a href="http://dev.mysql.com/doc/mysql/en/News.html" target="_top">http://dev.mysql.com/doc/mysql/en/News.html</a>)
can be particularly useful since it is quite possible that a
newer version contains a solution to your problem.
</p></li><li><p>
Search in the bugs database at
<a href="http://bugs.mysql.com/" target="_top">http://bugs.mysql.com/</a> to see whether the bug
has been reported and fixed.
</p></li><li><p>
Search the MySQL mailing list archives at
<a href="http://lists.mysql.com/" target="_top">http://lists.mysql.com/</a>.
</p></li><li><p>
You can also use <a href="http://www.mysql.com/search/" target="_top">http://www.mysql.com/search/</a>
to search all the Web pages (including the manual) that are
located at the MySQL AB Web site.
</p></li></ul></div><p>
If you can't find an answer in the manual or the archives, check
with your local MySQL expert. If you still can't find an answer
to your question, please follow the guidelines on sending mail
to a MySQL mailing list, outlined in the next section, before
contacting us.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="bug-reports"></a>1.7.1.3. How to Report Bugs or Problems</h4></div></div></div><a class="indexterm" name="id2762074"></a><a class="indexterm" name="id2762087"></a><a class="indexterm" name="id2762099"></a><a class="indexterm" name="id2762112"></a><a class="indexterm" name="id2762124"></a><a class="indexterm" name="id2762135"></a><a class="indexterm" name="id2762148"></a><a class="indexterm" name="id2762161"></a><a class="indexterm" name="id2762170"></a><p>
The normal place to report bugs is
<a href="http://bugs.mysql.com/" target="_top">http://bugs.mysql.com/</a>, which is the address for
our bugs database. This database is public, and can be browsed
and searched by anyone. If you log in to the system, you can
enter new reports.
</p><p>
Writing a good bug report takes patience, but doing it right the
first time saves time both for us and for yourself. A good bug
report, containing a full test case for the bug, makes it very
likely that we will fix the bug in the next release. This
section helps you write your report correctly so that you don't
waste your time doing things that may not help us much or at
all.
</p><p>
We encourage everyone to use the <span><strong class="command">mysqlbug</strong></span>
script to generate a bug report (or a report about any problem).
<span><strong class="command">mysqlbug</strong></span> can be found in the
<code class="filename">scripts</code> directory (source distribution) and
in the <code class="filename">bin</code> directory under your MySQL
installation directory (binary distribution). If you are unable
to use <span><strong class="command">mysqlbug</strong></span> (for example, if you are
running on Windows), it is still vital that you include all the
necessary information noted in this section (most importantly, a
description of the operating system and the MySQL version).
</p><p>
The <span><strong class="command">mysqlbug</strong></span> script helps you generate a
report by determining much of the following information
automatically, but if something important is missing, please
include it with your message. Please read this section carefully
and make sure that all the information described here is
included in your report.
</p><p>
Preferably, you should test the problem using the latest
production or development version of MySQL Server before
posting. Anyone should be able to repeat the bug by just using
<code class="literal">mysql test < script_file</code> on the included
test case or by running the shell or Perl script that is
included in the bug report.
</p><p>
All bugs posted in the bugs database at
<a href="http://bugs.mysql.com/" target="_top">http://bugs.mysql.com/</a> are corrected or
documented in the next MySQL release. If only minor code changes
are needed to correct a problem, we may also post a patch that
fixes the problem.
</p><p>
If you have found a sensitive security bug in MySQL, you can
send email to <code class="email"><<a href="mailto:security@mysql.com">security@mysql.com</a>></code>.
</p><p>
If you have a repeatable bug report, please report it to the
bugs database at <a href="http://bugs.mysql.com/" target="_top">http://bugs.mysql.com/</a>. Note
that even in this case it's good to run the
<span><strong class="command">mysqlbug</strong></span> script first to find information
about your system. Any bug that we are able to repeat has a high
chance of being fixed in the next MySQL release.
</p><p>
To report other problems, you can use one of the MySQL mailing
lists.
</p><p>
Remember that it is possible for us to respond to a message
containing too much information, but not to one containing too
little. People often omit facts because they think they know the
cause of a problem and assume that some details don't matter. A
good principle is this: If you are in doubt about stating
something, state it. It is faster and less troublesome to write
a couple more lines in your report than to wait longer for the
answer if we must ask you to provide information that was
missing from the initial report.
</p><p>
The most common errors made in bug reports are (a) not including
the version number of the MySQL distribution used, and (b) not
fully describing the platform on which the MySQL server is
installed (including the platform type and version number). This
is highly relevant information, and in 99 cases out of 100, the
bug report is useless without it. Very often we get questions
like, “<span class="quote">Why doesn't this work for me?</span>” Then we find
that the feature requested wasn't implemented in that MySQL
version, or that a bug described in a report has been fixed in
newer MySQL versions. Sometimes the error is platform-dependent;
in such cases, it is next to impossible for us to fix anything
without knowing the operating system and the version number of
the platform.
</p><p>
If you compiled MySQL from source, remember also to provide
information about your compiler, if it is related to the
problem. Often people find bugs in compilers and think the
problem is MySQL-related. Most compilers are under development
all the time and become better version by version. To determine
whether your problem depends on your compiler, we need to know
what compiler you use. Note that every compiling problem should
be regarded as a bug and reported accordingly.
</p><p>
It is most helpful when a good description of the problem is
included in the bug report. That is, give a good example of
everything you did that led to the problem and describe, in
exact detail, the problem itself. The best reports are those
that include a full example showing how to reproduce the bug or
problem. See <a href="porting.html#reproduceable-test-case" title="E.1.6. Making a Test Case If You Experience Table Corruption">Section E.1.6, “Making a Test Case If You Experience Table Corruption”</a>.
</p><p>
If a program produces an error message, it is very important to
include the message in your report. If we try to search for
something from the archives using programs, it is better that
the error message reported exactly matches the one that the
program produces. (Even the lettercase should be observed.) You
should never try to reproduce from memory what the error message
was; instead, copy and paste the entire message into your
report.
</p><p>
If you have a problem with Connector/ODBC (MyODBC), please try
to generate a trace file and send it with your report. See
<a href="connectors.html#myodbc-bug-report" title="23.1.1.9. How to Report MyODBC Problems or Bugs">Section 23.1.1.9, “How to Report MyODBC Problems or Bugs”</a>.
</p><p>
Please remember that many of the people who read your report do
so using an 80-column display. When generating reports or
examples using the <span><strong class="command">mysql</strong></span> command-line tool,
you should therefore use the <code class="option">--vertical</code> option
(or the <code class="literal">\G</code> statement terminator) for output
that would exceed the available width for such a display (for
example, with the <code class="literal">EXPLAIN SELECT</code> statement;
see the example later in this section).
</p><a class="indexterm" name="id2762411"></a><p>
Please include the following information in your report:
</p><div class="itemizedlist"><ul type="disc"><li><p>
The version number of the MySQL distribution you are using
(for example, MySQL 4.0.12). You can find out which version
you are running by executing <span><strong class="command">mysqladmin
version</strong></span>. The <span><strong class="command">mysqladmin</strong></span> program
can be found in the <code class="filename">bin</code> directory under
your MySQL installation directory.
</p></li><li><p>
The manufacturer and model of the machine on which you
experience the problem.
</p></li><li><p>
The operating system name and version. If you work with
Windows, you can usually get the name and version number by
double-clicking your My Computer icon and pulling down the
“<span class="quote">Help/About Windows</span>” menu. For most Unix-like
operating systems, you can get this information by executing
the command <code class="literal">uname -a</code>.
</p></li><li><p>
Sometimes the amount of memory (real and virtual) is
relevant. If in doubt, include these values.
</p></li><li><p>
If you are using a source distribution of the MySQL
software, the name and version number of the compiler used
are needed. If you have a binary distribution, the
distribution name is needed.
</p></li><li><p>
If the problem occurs during compilation, include the exact
error messages and also a few lines of context around the
offending code in the file where the error occurs.
</p></li><li><p>
If <span><strong class="command">mysqld</strong></span> died, you should also report
the query that crashed <span><strong class="command">mysqld</strong></span>. You can
usually find this out by running <span><strong class="command">mysqld</strong></span>
with query logging enabled, and then looking in the log
after <span><strong class="command">mysqld</strong></span> crashes See
<a href="porting.html#using-log-files" title="E.1.5. Using Log Files to Find Cause of Errors in mysqld">Section E.1.5, “Using Log Files to Find Cause of Errors in <span><strong class="command">mysqld</strong></span>”</a>.
</p></li><li><p>
If a database table is related to the problem, include the
output from <span><strong class="command">mysqldump --no-data
<em class="replaceable"><code>db_name</code></em>
<em class="replaceable"><code>tbl_name</code></em></strong></span>. This is very
easy to do and is a powerful way to get information about
any table in a database. The information helps us create a
situation matching the one you have experienced.
</p></li><li><p>
For speed-related bugs or problems with
<code class="literal">SELECT</code> statements, you should always
include the output of <code class="literal">EXPLAIN SELECT ...</code>,
and at least the number of rows that the
<code class="literal">SELECT</code> statement produces. You should
also include the output from <code class="literal">SHOW CREATE TABLE
<em class="replaceable"><code>tbl_name</code></em></code> for each
involved table. The more information you give about your
situation, the more likely it is that someone can help you.
</p><p>
The following is an example of a very good bug report. It
should be posted with the <span><strong class="command">mysqlbug</strong></span>
script. The example uses the <span><strong class="command">mysql</strong></span>
command-line tool. Note the use of the <code class="literal">\G</code>
statement terminator for statements whose output width would
otherwise exceed that of an 80-column display device.
</p><pre class="programlisting">mysql> <strong class="userinput"><code>SHOW VARIABLES;</code></strong>
mysql> <strong class="userinput"><code>SHOW COLUMNS FROM ...\G</code></strong>
<output from SHOW COLUMNS>
mysql> <strong class="userinput"><code>EXPLAIN SELECT ...\G</code></strong>
<output from EXPLAIN>
mysql> <strong class="userinput"><code>FLUSH STATUS;</code></strong>
mysql> <strong class="userinput"><code>SELECT ...;</code></strong>
<A short version of the output from SELECT,
including the time taken to run the query>
mysql> <strong class="userinput"><code>SHOW STATUS;</code></strong>
<output from SHOW STATUS>
</pre></li><li><p>
If a bug or problem occurs while running
<span><strong class="command">mysqld</strong></span>, try to provide an input script
that reproduces the anomaly. This script should include any
necessary source files. The more closely the script can
reproduce your situation, the better. If you can make a
reproducible test case, you should post it on
<a href="http://bugs.mysql.com/" target="_top">http://bugs.mysql.com/</a> for high-priority
treatment.
</p><p>
If you can't provide a script, you should at least include
the output from <span><strong class="command">mysqladmin variables
extended-status processlist</strong></span> in your mail to
provide some information on how your system is performing.
</p></li><li><p>
If you can't produce a test case with only a few rows, or if
the test table is too big to be mailed to the mailing list
(more than 10 rows), you should dump your tables using
<span><strong class="command">mysqldump</strong></span> and create a
<code class="filename">README</code> file that describes your
problem.
</p><p>
Create a compressed archive of your files using
<span><strong class="command">tar</strong></span> and <span><strong class="command">gzip</strong></span> or
<span><strong class="command">zip</strong></span>, and use FTP to transfer the archive
to <a href="ftp://ftp.mysql.com/pub/mysql/upload/" target="_top">ftp://ftp.mysql.com/pub/mysql/upload/</a>.
Then enter the problem into our bugs database at
<a href="http://bugs.mysql.com/" target="_top">http://bugs.mysql.com/</a>.
</p></li><li><p>
If you think that the MySQL server produces a strange result
from a query, include not only the result, but also your
opinion of what the result should be, and an account
describing the basis for your opinion.
</p></li><li><p>
When giving an example of the problem, it's better to use
the variable names, table names, and so on that exist in
your actual situation than to come up with new names. The
problem could be related to the name of a variable or table.
These cases are rare, perhaps, but it is better to be safe
than sorry. After all, it should be easier for you to
provide an example that uses your actual situation, and it
is by all means better for us. In case you have data that
you don't want to show to others, you can use FTP to
transfer it to
<a href="ftp://ftp.mysql.com/pub/mysql/upload/" target="_top">ftp://ftp.mysql.com/pub/mysql/upload/</a>. If the
information is really top secret and you don't want to show
it even to us, then go ahead and provide an example using
other names, but please regard this as the last choice.
</p></li><li><p>
Include all the options given to the relevant programs, if
possible. For example, indicate the options that you use
when you start the <span><strong class="command">mysqld</strong></span> server as well
as the options that you use to run any MySQL client
programs. The options to programs such as
<span><strong class="command">mysqld</strong></span> and <span><strong class="command">mysql</strong></span>, and
to the <span><strong class="command">configure</strong></span> script, are often keys
to answers and are very relevant. It is never a bad idea to
include them. If you use any modules, such as Perl or PHP,
please include the version numbers of those as well.
</p></li><li><p>
If your question is related to the privilege system, please
include the output of <span><strong class="command">mysqlaccess</strong></span>, the
output of <span><strong class="command">mysqladmin reload</strong></span>, and all the
error messages you get when trying to connect. When you test
your privileges, you should first run
<span><strong class="command">mysqlaccess</strong></span>. After this, execute
<span><strong class="command">mysqladmin reload version</strong></span> and try to
connect with the program that gives you trouble.
<span><strong class="command">mysqlaccess</strong></span> can be found in the
<code class="filename">bin</code> directory under your MySQL
installation directory.
</p></li><li><p>
If you have a patch for a bug, do include it. But don't
assume that the patch is all we need, or that we can use it,
if you don't provide some necessary information such as test
cases showing the bug that your patch fixes. We might find
problems with your patch or we might not understand it at
all; if so, we can't use it.
</p><p>
If we can't verify exactly what the purpose of the patch is,
we won't use it. Test cases help us here. Show that the
patch handles all the situations that may occur. If we find
a borderline case (even a rare one) where the patch won't
work, it may be useless.
</p></li><li><p>
Guesses about what the bug is, why it occurs, or what it
depends on are usually wrong. Even the MySQL team can't
guess such things without first using a debugger to
determine the real cause of a bug.
</p></li><li><p>
Indicate in your bug report that you have checked the
reference manual and mail archive so that others know you
have tried to solve the problem yourself.
</p></li><li><p>
If you get a <code class="literal">parse error</code>, please check
your syntax closely. If you can't find something wrong with
it, it's extremely likely that your current version of MySQL
Server doesn't support the syntax you are using. If you are
using the current version and the manual at
<a href="http://dev.mysql.com/doc/" target="_top">http://dev.mysql.com/doc/</a> doesn't cover the
syntax you are using, MySQL Server doesn't support your
query. In this case, your only options are to implement the
syntax yourself or email <code class="email"><<a href="mailto:licensing@mysql.com">licensing@mysql.com</a>></code>
and ask for an offer to implement it.
</p><p>
If the manual covers the syntax you are using, but you have
an older version of MySQL Server, you should check the MySQL
change history to see when the syntax was implemented. In
this case, you have the option of upgrading to a newer
version of MySQL Server. See <a href="news.html" title="Appendix D. MySQL Change History">Appendix D, <i>MySQL Change History</i></a>.
</p></li><li><p>
If your problem is that your data appears corrupt or you get
errors when you access a particular table, you should first
check and then try to repair your tables with <code class="literal">CHECK
TABLE</code> and <code class="literal">REPAIR TABLE</code> or with
<span><strong class="command">myisamchk</strong></span>. See
<a href="database-administration.html" title="Chapter 5. Database Administration">Chapter 5, <i>Database Administration</i></a>.
</p><p>
If you are running Windows, please verify the value of
<code class="literal">lower_case_table_names</code> using the
<code class="literal">SHOW VARIABLES LIKE
'lower_case_table_names'</code> command.
</p></li><li><p>
If you often get corrupted tables, you should try to find
out when and why this happens. In this case, the error log
in the MySQL data directory may contain some information
about what happened. (This is the file with the
<code class="filename">.err</code> suffix in the name.) See
<a href="database-administration.html#error-log" title="5.11.1. The Error Log">Section 5.11.1, “The Error Log”</a>. Please include any relevant
information from this file in your bug report. Normally
<span><strong class="command">mysqld</strong></span> should <span class="emphasis"><em>never</em></span>
crash a table if nothing killed it in the middle of an
update. If you can find the cause of
<span><strong class="command">mysqld</strong></span> dying, it's much easier for us to
provide you with a fix for the problem. See
<a href="problems.html#what-is-crashing" title="A.1. How to Determine What Is Causing a Problem">Section A.1, “How to Determine What Is Causing a Problem”</a>.
</p></li><li><p>
If possible, download and install the most recent version of
MySQL Server and check whether it solves your problem. All
versions of the MySQL software are thoroughly tested and
should work without problems. We believe in making
everything as backward-compatible as possible, and you
should be able to switch MySQL versions without difficulty.
See <a href="installing.html#which-version" title="2.1.2. Choosing Which MySQL Distribution to Install">Section 2.1.2, “Choosing Which MySQL Distribution to Install”</a>.
</p></li></ul></div><a class="indexterm" name="id2763020"></a><a class="indexterm" name="id2763032"></a><a class="indexterm" name="id2763045"></a><a class="indexterm" name="id2763057"></a><p>
If you are a support customer, please cross-post the bug report
to <code class="email"><<a href="mailto:mysql-support@mysql.com">mysql-support@mysql.com</a>></code> for higher-priority
treatment, as well as to the appropriate mailing list to see
whether someone else has experienced (and perhaps solved) the
problem.
</p><p>
For information on reporting bugs in MyODBC, see
<a href="connectors.html#myodbc-bug-report" title="23.1.1.9. How to Report MyODBC Problems or Bugs">Section 23.1.1.9, “How to Report MyODBC Problems or Bugs”</a>.
</p><p>
For solutions to some common problems, see
<a href="problems.html" title="Appendix A. Problems and Common Errors">Appendix A, <i>Problems and Common Errors</i></a>.
</p><p>
When answers are sent to you individually and not to the mailing
list, it is considered good etiquette to summarize the answers
and send the summary to the mailing list so that others may have
the benefit of responses you received that helped you solve your
problem.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="answering-questions"></a>1.7.1.4. Guidelines for Answering Questions on the Mailing List</h4></div></div></div><a class="indexterm" name="id2763121"></a><a class="indexterm" name="id2763130"></a><a class="indexterm" name="id2763143"></a><a class="indexterm" name="id2763155"></a><p>
If you consider your answer to have broad interest, you may want
to post it to the mailing list instead of replying directly to
the individual who asked. Try to make your answer general enough
that people other than the original poster may benefit from it.
When you post to the list, please make sure that your answer is
not a duplication of a previous answer.
</p><p>
Try to summarize the essential part of the question in your
reply; don't feel obliged to quote the entire original message.
</p><p>
Please don't post mail messages from your browser with HTML mode
turned on. Many users don't read mail with a browser.
</p></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="irc"></a>1.7.2. MySQL Community Support on IRC (Internet Relay Chat)</h3></div></div></div><a class="indexterm" name="id2763202"></a><a class="indexterm" name="id2763211"></a><p>
In addition to the various MySQL mailing lists, you can find
experienced community people on <code class="literal">IRC</code>
(<code class="literal">Internet Relay Chat</code>). These are the best
networks/channels currently known to us:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<span class="bold"><strong>freenode</strong></span> (see
<a href="http://www.freenode.net/" target="_top">http://www.freenode.net/</a> for servers)
</p><div class="itemizedlist"><ul type="circle"><li><p>
<code class="literal">#mysql</code> Primarily MySQL questions, but
other database and general SQL questions are welcome.
Questions about PHP, Perl or C in combination with MySQL
are also common.
</p></li></ul></div></li></ul></div><p>
If you are looking for IRC client software to connect to an IRC
network, take a look at <code class="literal">xChat</code>
(<a href="http://www.xchat.org/" target="_top">http://www.xchat.org/</a>). X-Chat (GPL licensed) is
available for Unix as well as for Windows platforms (a free
Windows build of X-Chat is available at
<a href="http://www.silverex.org/download/" target="_top">http://www.silverex.org/download/</a>).
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="forums"></a>1.7.3. MySQL Community Support at the MySQL Forums</h3></div></div></div><a class="indexterm" name="id2763312"></a><p>
The latest community support resource are the forums at
<a href="http://forums.mysql.com" target="_top">http://forums.mysql.com</a>.
</p><p>
There are a variety of forums available, grouped in the following
general categories:
</p><div class="itemizedlist"><ul type="disc"><li><p>
Migration
</p></li><li><p>
MySQL Usage
</p></li><li><p>
MySQL Connectors
</p></li><li><p>
Programming Languages
</p></li><li><p>
Tools
</p></li><li><p>
3rd-Party Applications
</p></li><li><p>
Storage Engines
</p></li><li><p>
MySQL Technology
</p></li><li><p>
SQL Standards
</p></li><li><p>
Business
</p></li></ul></div></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="compatibility"></a>1.8. MySQL Standards Compliance</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="introduction.html#standards">1.8.1. What Standards MySQL Follows</a></span></dt><dt><span class="section"><a href="introduction.html#sql-mode">1.8.2. Selecting SQL Modes</a></span></dt><dt><span class="section"><a href="introduction.html#ansi-mode">1.8.3. Running MySQL in ANSI Mode</a></span></dt><dt><span class="section"><a href="introduction.html#extensions-to-ansi">1.8.4. MySQL Extensions to Standard SQL</a></span></dt><dt><span class="section"><a href="introduction.html#differences-from-ansi">1.8.5. MySQL Differences from Standard SQL</a></span></dt><dt><span class="section"><a href="introduction.html#constraints">1.8.6. How MySQL Deals with Constraints</a></span></dt></dl></div><a class="indexterm" name="id2763422"></a><a class="indexterm" name="id2763432"></a><a class="indexterm" name="id2763439"></a><a class="indexterm" name="id2763450"></a><a class="indexterm" name="id2763460"></a><p>
This section describes how MySQL relates to the ANSI/ISO SQL
standards. MySQL Server has many extensions to the SQL standard,
and here you can find out what they are and how to use them. You
can also find information about functionality missing from MySQL
Server, and how to work around some differences.
</p><p>
The SQL standard has been evolving since 1986 and several versions
exist. In this manual, “<span class="quote">SQL-92</span>” refers to the
standard released in 1992, “<span class="quote">SQL:1999</span>” refers to the
standard released in 1999, and “<span class="quote">SQL:2003</span>” refers to
the current version of the standard. We use the phrase “<span class="quote">the
SQL standard</span>” to mean the current version of the SQL
Standard at any time.
</p><p>
Our goal is to not restrict MySQL Server usability for any usage
without a very good reason for doing so. Even if we don't have the
resources to perform development for every possible use, we are
always willing to help and offer suggestions to people who are
trying to use MySQL Server in new territories.
</p><p>
One of our main goals with the product is to continue to work
toward compliance with the SQL standard, but without sacrificing
speed or reliability. We are not afraid to add extensions to SQL
or support for non-SQL features if this greatly increases the
usability of MySQL Server for a large segment of our user base.
The <code class="literal">HANDLER</code> interface in MySQL Server 4.0 is an
example of this strategy. See <a href="sql-syntax.html#handler" title="13.2.3. HANDLER Syntax">Section 13.2.3, “<code class="literal">HANDLER</code> Syntax”</a>.
</p><p>
We continue to support transactional and non-transactional
databases to satisfy both mission-critical 24/7 usage and heavy
Web or logging usage.
</p><p>
MySQL Server was originally designed to work with medium size
databases (10-100 million rows, or about 100MB per table) on small
computer systems. Today MySQL Server handles terabyte-size
databases, but the code can also be compiled in a reduced version
suitable for hand-held and embedded devices. The compact design of
the MySQL server makes development in both directions possible
without any conflicts in the source tree.
</p><p>
Currently, we are not targeting realtime support, although MySQL
replication capabilities offer significant functionality.
</p><p>
Database cluster support exists through third-party clustering
solutions as well as the integration of our acquired NDB Cluster
technology, available from version 4.1.2. See
<a href="ndbcluster.html" title="Chapter 15. MySQL Cluster">Chapter 15, <i>MySQL Cluster</i></a>.
</p><p>
We are also looking at providing XML support in the database
server.
</p><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="standards"></a>1.8.1. What Standards MySQL Follows</h3></div></div></div><p>
We are aiming toward supporting the full ANSI/ISO SQL standard,
but without making concessions to speed and quality of the code.
</p><p>
ODBC levels 0-3.51.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="sql-mode"></a>1.8.2. Selecting SQL Modes</h3></div></div></div><p>
The MySQL server can operate in different SQL modes, and can
apply these modes differentially for different clients. This
allows an application to tailor server operation to its own
requirements.
</p><p>
Modes define what SQL syntax MySQL should support and what kind
of validation checks it should perform on the data. This makes
it easier to use MySQL in a lot of different environments and to
use MySQL together with other database servers.
</p><p>
You can set the default SQL mode by starting
<span><strong class="command">mysqld</strong></span> with the
<code class="option">--sql-mode="modes"</code> option. Beginning with MySQL
4.1, you can also change the mode after startup time by setting
the <code class="literal">sql_mode</code> variable with a <code class="literal">SET
[SESSION|GLOBAL] sql_mode='modes'</code> statement.
</p><p>
For more information on setting the server mode, see
<a href="database-administration.html#server-sql-mode" title="5.3.2. The Server SQL Mode">Section 5.3.2, “The Server SQL Mode”</a>.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="ansi-mode"></a>1.8.3. Running MySQL in ANSI Mode</h3></div></div></div><a class="indexterm" name="id2763661"></a><a class="indexterm" name="id2763671"></a><p>
You can tell <span><strong class="command">mysqld</strong></span> to use the ANSI mode with
the <code class="option">--ansi</code> startup option. See
<a href="database-administration.html#server-options" title="5.3.1. mysqld Command-Line Options">Section 5.3.1, “<span><strong class="command">mysqld</strong></span> Command-Line Options”</a>.
</p><p>
Running the server in ANSI mode is the same as starting it with
these options (specify the <code class="option">--sql_mode</code> value on
a single line):
</p><pre class="programlisting">--transaction-isolation=SERIALIZABLE
--sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
IGNORE_SPACE
</pre><p>
In MySQL 4.1, you can achieve the same effect with these two
statements (specify the <code class="literal">sql_mode</code> value on a
single line):
</p><pre class="programlisting">SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET GLOBAL sql_mode = 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
IGNORE_SPACE';
</pre><p>
See <a href="introduction.html#sql-mode" title="1.8.2. Selecting SQL Modes">Section 1.8.2, “Selecting SQL Modes”</a>.
</p><p>
In MySQL 4.1.1, the <code class="literal">sql_mode</code> options shown
can be also be set with this statement:
</p><pre class="programlisting">SET GLOBAL sql_mode='ansi';
</pre><p>
In this case, the value of the <code class="literal">sql_mode</code>
variable is set to all options that are relevant for ANSI mode.
You can check the result like this:
</p><pre class="programlisting">mysql> <strong class="userinput"><code>SET GLOBAL sql_mode='ansi';</code></strong>
mysql> <strong class="userinput"><code>SELECT @@global.sql_mode;</code></strong>
-> 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,
IGNORE_SPACE,ANSI';
</pre></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="extensions-to-ansi"></a>1.8.4. MySQL Extensions to Standard SQL</h3></div></div></div><a class="indexterm" name="id2763785"></a><p>
The MySQL Server includes some extensions that you probably
won't find in other SQL DBMSs. Be warned that if you use them,
your code won't be portable to other SQL servers. In some cases,
you can write code that includes MySQL extensions, but is still
portable, by using comments of the form <code class="literal">/*! ...
*/</code>. In this case, MySQL Server parses and execute the
code within the comment as it would any other MySQL statement,
but other SQL servers will ignore the extensions. For example:
</p><pre class="programlisting">SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...
</pre><p>
If you add a version number after the
‘<code class="literal">!</code>’ character, the syntax within
the comment is executed only if the MySQL version is equal to or
newer than the specified version number:
</p><pre class="programlisting">CREATE /*!32302 TEMPORARY */ TABLE t (a INT);
</pre><p>
This means that if you have Version 3.23.02 or newer, MySQL
Server uses the <code class="literal">TEMPORARY</code> keyword.
</p><p>
The following descriptions list MySQL extensions, organized by
category.
</p><div class="itemizedlist"><ul type="disc"><li><p>
Organization of data on disk
</p><p>
MySQL Server maps each database to a directory under the
MySQL data directory, and tables within a database to
filenames in the database directory. This has a few
implications:
</p><div class="itemizedlist"><ul type="circle"><li><p>
<a class="indexterm" name="id2763863"></a>
<a class="indexterm" name="id2763874"></a>
<a class="indexterm" name="id2763884"></a>
<a class="indexterm" name="id2763894"></a>
Database names and table names are case sensitive in
MySQL Server on operating systems that have
case-sensitive filenames (such as most Unix systems).
See <a href="language-structure.html#name-case-sensitivity" title="9.2.2. Identifier Case Sensitivity">Section 9.2.2, “Identifier Case Sensitivity”</a>.
</p></li><li><p>
You can use standard system commands to back up, rename,
move, delete, and copy tables that are managed by the
<code class="literal">MyISAM</code> or <code class="literal">ISAM</code>
storage engines. For example, to rename a
<code class="literal">MyISAM</code> table, rename the
<code class="filename">.MYD</code>, <code class="filename">.MYI</code>,
and <code class="filename">.frm</code> files to which the table
corresponds.
</p></li></ul></div><p>
Database, table, index, column, or alias names may begin
with a digit (but may not consist solely of digits).
</p></li><li><p>
General language syntax
</p><div class="itemizedlist"><ul type="circle"><li><p>
Strings may be enclosed by either
‘<code class="literal">"</code>’ or
‘<code class="literal">'</code>’, not just by
‘<code class="literal">'</code>’.
</p></li><li><p>
Use of ‘<code class="literal">\</code>’ as an escape
character in strings.
</p></li><li><p>
In SQL statements, you can access tables from different
databases with the
<em class="replaceable"><code>db_name.tbl_name</code></em> syntax. Some
SQL servers provide the same functionality but call this
<code class="literal">User space</code>. MySQL Server doesn't
support tablespaces such as used in statements like
this: <code class="literal">CREATE TABLE ralph.my_table...IN
my_tablespace</code>.
</p></li></ul></div></li><li><p>
SQL statement syntax
</p><div class="itemizedlist"><ul type="circle"><li><p>
The <code class="literal">ANALYZE TABLE</code>, <code class="literal">CHECK
TABLE</code>, <code class="literal">OPTIMIZE TABLE</code>, and
<code class="literal">REPAIR TABLE</code> statements.
</p></li><li><p>
The <code class="literal">CREATE DATABASE</code> and <code class="literal">DROP
DATABASE</code> statements. See
<a href="sql-syntax.html#create-database" title="13.1.3. CREATE DATABASE Syntax">Section 13.1.3, “<code class="literal">CREATE DATABASE</code> Syntax”</a>.
</p></li><li><p>
The <code class="literal">DO</code> statement.
</p></li><li><p>
<code class="literal">EXPLAIN SELECT</code> to get a description
of how tables are joined.
</p></li><li><p>
The <code class="literal">FLUSH</code> and
<code class="literal">RESET</code> statements.
</p></li><li><p>
The <code class="literal">SET</code> statement. See
<a href="sql-syntax.html#set-option" title="13.5.3. SET Syntax">Section 13.5.3, “<code class="literal">SET</code> Syntax”</a>.
</p></li><li><p>
The <code class="literal">SHOW</code> statement. See
<a href="sql-syntax.html#show" title="13.5.4. SHOW Syntax">Section 13.5.4, “<code class="literal">SHOW</code> Syntax”</a>.
</p><a class="indexterm" name="id2764144"></a><a class="indexterm" name="id2764151"></a></li><li><p>
Use of <code class="literal">LOAD DATA INFILE</code>. In many
cases, this syntax is compatible with Oracle's
<code class="literal">LOAD DATA INFILE</code>. See
<a href="sql-syntax.html#load-data" title="13.2.5. LOAD DATA INFILE Syntax">Section 13.2.5, “<code class="literal">LOAD DATA INFILE</code> Syntax”</a>.
</p></li><li><p>
Use of <code class="literal">RENAME TABLE</code>. See
<a href="sql-syntax.html#rename-table" title="13.1.9. RENAME TABLE Syntax">Section 13.1.9, “<code class="literal">RENAME TABLE</code> Syntax”</a>.
</p></li><li><p>
Use of <code class="literal">REPLACE</code> instead of
<code class="literal">DELETE</code> + <code class="literal">INSERT</code>.
See <a href="sql-syntax.html#replace" title="13.2.6. REPLACE Syntax">Section 13.2.6, “<code class="literal">REPLACE</code> Syntax”</a>.
</p></li><li><p>
Use of <code class="literal">CHANGE col_name</code>, <code class="literal">DROP
col_name</code>, or <code class="literal">DROP INDEX</code>,
<code class="literal">IGNORE</code> or <code class="literal">RENAME</code>
in an <code class="literal">ALTER TABLE</code> statement. Use of
multiple <code class="literal">ADD</code>,
<code class="literal">ALTER</code>, <code class="literal">DROP</code>, or
<code class="literal">CHANGE</code> clauses in an <code class="literal">ALTER
TABLE</code> statement. See
<a href="sql-syntax.html#alter-table" title="13.1.2. ALTER TABLE Syntax">Section 13.1.2, “<code class="literal">ALTER TABLE</code> Syntax”</a>.
</p></li><li><p>
Use of index names, indexes on a prefix of a field, and
use of <code class="literal">INDEX</code> or
<code class="literal">KEY</code> in a <code class="literal">CREATE
TABLE</code> statement. See
<a href="sql-syntax.html#create-table" title="13.1.5. CREATE TABLE Syntax">Section 13.1.5, “<code class="literal">CREATE TABLE</code> Syntax”</a>.
</p></li><li><p>
Use of <code class="literal">TEMPORARY</code> or <code class="literal">IF NOT
EXISTS</code> with <code class="literal">CREATE TABLE</code>.
</p></li><li><p>
Use of <code class="literal">IF EXISTS</code> with <code class="literal">DROP
TABLE</code>.
</p></li><li><p>
You can drop multiple tables with a single <code class="literal">DROP
TABLE</code> statement.
</p></li><li><p>
The <code class="literal">ORDER BY</code> and
<code class="literal">LIMIT</code> clauses of the
<code class="literal">UPDATE</code> and <code class="literal">DELETE</code>
statements.
</p></li><li><p>
<code class="literal">INSERT INTO ... SET col_name = ...</code>
syntax.
</p></li><li><p>
The <code class="literal">DELAYED</code> clause of the
<code class="literal">INSERT</code> and <code class="literal">REPLACE</code>
statements.
</p></li><li><p>
The <code class="literal">LOW_PRIORITY</code> clause of the
<code class="literal">INSERT</code>, <code class="literal">REPLACE</code>,
<code class="literal">DELETE</code>, and <code class="literal">UPDATE</code>
statements.
</p></li><li><p>
Use of <code class="literal">INTO OUTFILE</code> and
<code class="literal">STRAIGHT_JOIN</code> in a
<code class="literal">SELECT</code> statement. See
<a href="sql-syntax.html#select" title="13.2.7. SELECT Syntax">Section 13.2.7, “<code class="literal">SELECT</code> Syntax”</a>.
</p></li><li><p>
The <code class="literal">SQL_SMALL_RESULT</code> option in a
<code class="literal">SELECT</code> statement.
</p></li><li><p>
You don't need to name all selected columns in the
<code class="literal">GROUP BY</code> part. This gives better
performance for some very specific, but quite normal
queries. See
<a href="functions.html#group-by-functions-and-modifiers" title="12.10. Functions and Modifiers for Use with GROUP BY Clauses">Section 12.10, “Functions and Modifiers for Use with <code class="literal">GROUP BY</code> Clauses”</a>.
</p></li><li><p>
You can specify <code class="literal">ASC</code> and
<code class="literal">DESC</code> with <code class="literal">GROUP
BY</code>.
</p></li><li><p>
The ability to set variables in a statement with the
<code class="literal">:=</code> assignment operator:
</p><pre class="programlisting">mysql> <strong class="userinput"><code>SELECT @a:=SUM(total),@b=COUNT(*),@a/@b AS avg</code></strong>
-> <strong class="userinput"><code>FROM test_table;</code></strong>
mysql> <strong class="userinput"><code>SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;</code></strong>
</pre></li></ul></div></li><li><p>
Column types
</p><div class="itemizedlist"><ul type="circle"><li><p>
The column types <code class="literal">MEDIUMINT</code>,
<code class="literal">SET</code>, <code class="literal">ENUM</code>, and the
different <code class="literal">BLOB</code> and
<code class="literal">TEXT</code> types.
</p></li><li><p>
The column attributes <code class="literal">AUTO_INCREMENT</code>,
<code class="literal">BINARY</code>, <code class="literal">NULL</code>,
<code class="literal">UNSIGNED</code>, and
<code class="literal">ZEROFILL</code>.
</p></li></ul></div></li><li><p>
Functions and operators
</p><div class="itemizedlist"><ul type="circle"><li><p>
To make it easier for users who come from other SQL
environments, MySQL Server supports aliases for many
functions. For example, all string functions support
both standard SQL syntax and ODBC syntax.
</p></li><li><p>
MySQL Server understands the <code class="literal">||</code> and
<code class="literal">&&</code> operators to mean logical
OR and AND, as in the C programming language. In MySQL
Server, <code class="literal">||</code> and <code class="literal">OR</code>
are synonyms, as are <code class="literal">&&</code> and
<code class="literal">AND</code>. Because of this nice syntax,
MySQL Server doesn't support the standard SQL
<code class="literal">||</code> operator for string concatenation;
use <code class="literal">CONCAT()</code> instead. Because
<code class="literal">CONCAT()</code> takes any number of
arguments, it's easy to convert use of the
<code class="literal">||</code> operator to MySQL Server.
</p></li><li><p>
Use of <code class="literal">COUNT(DISTINCT list)</code> where
<code class="literal">list</code> has more than one element.
</p></li><li><p>
All string comparisons are case-insensitive by default,
with sort ordering determined by the current character
set (cp1252 Latin1 by default). If you don't like this,
you should declare your columns with the
<code class="literal">BINARY</code> attribute or use the
<code class="literal">BINARY</code> cast, which causes comparisons
to be done using the underlying character code values
rather then a lexical ordering.
</p><a class="indexterm" name="id2764706"></a><a class="indexterm" name="id2764712"></a></li><li><p>
The <code class="literal">%</code> operator is a synonym for
<code class="literal">MOD()</code>. That is, <code class="literal">N %
M</code> is equivalent to
<code class="literal">MOD(N,M)</code>. <code class="literal">%</code> is
supported for C programmers and for compatibility with
PostgreSQL.
</p></li><li><p>
The <code class="literal">=</code>, <code class="literal"><></code>,
<code class="literal"><=</code>,<code class="literal"><</code>,
<code class="literal">>=</code>,<code class="literal">></code>,
<code class="literal"><<</code>,
<code class="literal">>></code>,
<code class="literal"><=></code>, <code class="literal">AND</code>,
<code class="literal">OR</code>, or <code class="literal">LIKE</code>
operators may be used in column comparisons to the left
of the <code class="literal">FROM</code> in
<code class="literal">SELECT</code> statements. For example:
</p><pre class="programlisting">mysql> <strong class="userinput"><code>SELECT col1=1 AND col2=2 FROM <em class="replaceable"><code>tbl_name</code></em>;</code></strong>
</pre></li><li><p>
The <code class="literal">LAST_INSERT_ID()</code> function that
returns the most recent
<code class="literal">AUTO_INCREMENT</code> value. See
<a href="functions.html#information-functions" title="12.9.3. Information Functions">Section 12.9.3, “Information Functions”</a>.
</p></li><li><p>
<code class="literal">LIKE</code> is allowed on numeric columns.
</p></li><li><p>
The <code class="literal">REGEXP</code> and <code class="literal">NOT
REGEXP</code> extended regular expression operators.
</p></li><li><p>
<code class="literal">CONCAT()</code> or <code class="literal">CHAR()</code>
with one argument or more than two arguments. (In MySQL
Server, these functions can take any number of
arguments.)
</p></li><li><p>
The <code class="literal">BIT_COUNT()</code>,
<code class="literal">CASE</code>, <code class="literal">ELT()</code>,
<code class="literal">FROM_DAYS()</code>,
<code class="literal">FORMAT()</code>, <code class="literal">IF()</code>,
<code class="literal">PASSWORD()</code>,
<code class="literal">ENCRYPT()</code>, <code class="literal">MD5()</code>,
<code class="literal">ENCODE()</code>,
<code class="literal">DECODE()</code>,
<code class="literal">PERIOD_ADD()</code>,
<code class="literal">PERIOD_DIFF()</code>,
<code class="literal">TO_DAYS()</code>, and
<code class="literal">WEEKDAY()</code> functions.
</p></li><li><p>
Use of <code class="literal">TRIM()</code> to trim substrings.
Standard SQL supports removal of single characters only.
</p></li><li><p>
The <code class="literal">GROUP BY</code> functions
<code class="literal">STD()</code>, <code class="literal">BIT_OR()</code>,
<code class="literal">BIT_AND()</code>,
<code class="literal">BIT_XOR()</code>, and
<code class="literal">GROUP_CONCAT()</code>. See
<a href="functions.html#group-by-functions-and-modifiers" title="12.10. Functions and Modifiers for Use with GROUP BY Clauses">Section 12.10, “Functions and Modifiers for Use with <code class="literal">GROUP BY</code> Clauses”</a>.
</p></li></ul></div></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="differences-from-ansi"></a>1.8.5. MySQL Differences from Standard SQL</h3></div></div></div><div class="toc"><dl><dt><span class="section"><a href="introduction.html#ansi-diff-subqueries">1.8.5.1. Subqueries</a></span></dt><dt><span class="section"><a href="introduction.html#ansi-diff-select-into-table">1.8.5.2. <code class="literal">SELECT INTO TABLE</code></a></span></dt><dt><span class="section"><a href="introduction.html#ansi-diff-transactions">1.8.5.3. Transactions and Atomic Operations</a></span></dt><dt><span class="section"><a href="introduction.html#ansi-diff-triggers">1.8.5.4. Stored Procedures and Triggers</a></span></dt><dt><span class="section"><a href="introduction.html#ansi-diff-foreign-keys">1.8.5.5. Foreign Keys</a></span></dt><dt><span class="section"><a href="introduction.html#ansi-diff-views">1.8.5.6. Views</a></span></dt><dt><span class="section"><a href="introduction.html#ansi-diff-comments">1.8.5.7. '<code class="literal">--</code>' as the Start of a Comment</a></span></dt></dl></div><p>
We try to make MySQL Server follow the ANSI SQL standard and the
ODBC SQL standard, but MySQL Server performs operations
differently in some cases:
</p><div class="itemizedlist"><ul type="disc"><li><p>
For <code class="literal">VARCHAR</code> columns, trailing spaces are
removed when the value is stored. (Fixed in MySQL 5.0.3).
See <a href="problems.html#bugs" title="A.8. Known Issues in MySQL">Section A.8, “Known Issues in MySQL”</a>.
</p></li><li><p>
In some cases, <code class="literal">CHAR</code> columns are silently
converted to <code class="literal">VARCHAR</code> columns when you
define a table or alter its structure. (Fixed in MySQL
5.0.3). See <a href="sql-syntax.html#silent-column-changes" title="13.1.5.1. Silent Column Specification Changes">Section 13.1.5.1, “Silent Column Specification Changes”</a>.
</p></li><li><p>
Privileges for a table are not automatically revoked when
you delete a table. You must explicitly issue a
<code class="literal">REVOKE</code> statement to revoke privileges for
a table. See <a href="sql-syntax.html#grant" title="13.5.1.3. GRANT and REVOKE Syntax">Section 13.5.1.3, “<code class="literal">GRANT</code> and <code class="literal">REVOKE</code> Syntax”</a>.
</p></li><li><p>
The <code class="literal">CAST()</code> function does not support cast
to <code class="literal">REAL</code> or <code class="literal">BIGINT</code>. See
<a href="functions.html#cast-functions" title="12.8. Cast Functions and Operators">Section 12.8, “Cast Functions and Operators”</a>.
</p></li><li><p>
Standard SQL requires that a <code class="literal">HAVING</code>
clause in a <code class="literal">SELECT</code> statement be able to
refer to columns in the <code class="literal">GROUP BY</code> clause.
This cannot be done before MySQL 5.0.2.
</p></li></ul></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="ansi-diff-subqueries"></a>1.8.5.1. Subqueries</h4></div></div></div><p>
MySQL 4.1 supports subqueries and derived tables. A
“<span class="quote">subquery</span>” is a <code class="literal">SELECT</code>
statement nested within another statement. A “<span class="quote">derived
table</span>” (an unnamed view) is a subquery in the
<code class="literal">FROM</code> clause of another statement. See
<a href="sql-syntax.html#subqueries" title="13.2.8. Subquery Syntax">Section 13.2.8, “Subquery Syntax”</a>.
</p><p>
For MySQL versions older than 4.1, most subqueries can be
rewritten using joins or other methods. See
<a href="sql-syntax.html#rewriting-subqueries" title="13.2.8.11. Rewriting Subqueries as Joins for Earlier MySQL Versions">Section 13.2.8.11, “Rewriting Subqueries as Joins for Earlier MySQL Versions”</a> for examples that show
how to do this.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="ansi-diff-select-into-table"></a>1.8.5.2. <code class="literal">SELECT INTO TABLE</code></h4></div></div></div><a class="indexterm" name="id2765195"></a><p>
MySQL Server doesn't support the Sybase SQL extension:
<code class="literal">SELECT ... INTO TABLE ...</code>. Instead, MySQL
Server supports the standard SQL syntax <code class="literal">INSERT INTO
... SELECT ...</code>, which is basically the same thing.
See <a href="sql-syntax.html#insert-select" title="13.2.4.1. INSERT ... SELECT Syntax">Section 13.2.4.1, “<code class="literal">INSERT ... SELECT</code> Syntax”</a>.
</p><pre class="programlisting">INSERT INTO tbl_temp2 (fld_id)
SELECT tbl_temp1.fld_order_id
FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;
</pre><p>
Alternatively, you can use <code class="literal">SELECT INTO OUTFILE
...</code> or <code class="literal">CREATE TABLE ... SELECT</code>.
</p><p>
From version 5.0, MySQL supports <code class="literal">SELECT ...
INTO</code> with user variables. The same syntax may also
be used inside stored procedures using cursors and local
variables. See <a href="stored-procedures.html#select-into-statement" title="17.2.9.3. SELECT ... INTO Statement">Section 17.2.9.3, “<code class="literal">SELECT ... INTO</code> Statement”</a>.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="ansi-diff-transactions"></a>1.8.5.3. Transactions and Atomic Operations</h4></div></div></div><a class="indexterm" name="id2765270"></a><a class="indexterm" name="id2765279"></a><a class="indexterm" name="id2765288"></a><a class="indexterm" name="id2765298"></a><a class="indexterm" name="id2765305"></a><a class="indexterm" name="id2765315"></a><a class="indexterm" name="id2765325"></a><a class="indexterm" name="id2765334"></a><a class="indexterm" name="id2765343"></a><p>
MySQL Server (version 3.23-max and all versions 4.0 and above)
supports transactions with the <code class="literal">InnoDB</code> and
<code class="literal">BDB</code> transactional storage engines.
<code class="literal">InnoDB</code> provides <span class="emphasis"><em>full</em></span>
<code class="literal">ACID</code> compliance. See
<a href="storage-engines.html" title="Chapter 14. Storage Engines and Table Types">Chapter 14, <i>Storage Engines and Table Types</i></a>.
</p><p>
The other non-transactional storage engines in MySQL Server
(such as <code class="literal">MyISAM</code>) follow a different
paradigm for data integrity called “<span class="quote">atomic
operations.</span>” In transactional terms,
<code class="literal">MyISAM</code> tables effectively always operate in
<code class="literal">AUTOCOMMIT=1</code> mode. Atomic operations often
offer comparable integrity with higher performance.
</p><p>
With MySQL Server supporting both paradigms, you can decide
whether your applications are best served by the speed of
atomic operations or the use of transactional features. This
choice can be made on a per-table basis.
</p><p>
As noted, the trade-off for transactional versus
non-transactional table types lies mostly in performance.
Transactional tables have significantly higher memory and
diskspace requirements, and more CPU overhead. On the other
hand, transactional table types such as
<code class="literal">InnoDB</code> also offer many significant
features. MySQL Server's modular design allows the concurrent
use of different storage engines to suit different
requirements and deliver optimum performance in all
situations.
</p><p>
But how do you use the features of MySQL Server to maintain
rigorous integrity even with the non-transactional
<code class="literal">MyISAM</code> tables, and how do these features
compare with the transactional table types?
</p><div class="orderedlist"><ol type="1"><li><p>
If your applications are written in a way that is
dependent on being able to call
<code class="literal">ROLLBACK</code> rather than
<code class="literal">COMMIT</code> in critical situations,
transactions are more convenient. Transactions also ensure
that unfinished updates or corrupting activities are not
committed to the database; the server is given the
opportunity to do an automatic rollback and your database
is saved.
</p><p>
If you use non-transactional tables, MySQL Server in
almost all cases allows you to resolve potential problems
by including simple checks before updates and by running
simple scripts that check the databases for
inconsistencies and automatically repair or warn if such
an inconsistency occurs. Note that just by using the MySQL
log or even adding one extra log, you can normally fix
tables perfectly with no data integrity loss.
</p></li><li><p>
More often than not, critical transactional updates can be
rewritten to be atomic. Generally speaking, all integrity
problems that transactions solve can be done with
<code class="literal">LOCK TABLES</code> or atomic updates, ensuring
that there are no automatic aborts from the server, which
is a common problem with transactional database systems.
</p></li><li><p>
To be safe with MySQL Server, whether or not using
transactional tables, you only need to have backups and
have binary logging turned on. With this you can recover
from any situation that you could with any other
transactional database system. It is always good to have
backups, regardless of which database system you use.
</p></li></ol></div><p>
The transactional paradigm has its benefits and its drawbacks.
Many users and application developers depend on the ease with
which they can code around problems where an abort appears to
be, or is necessary. However, even if you are new to the
atomic operations paradigm, or more familiar with
transactions, do consider the speed benefit that
non-transactional tables can offer on the order of three to
five times the speed of the fastest and most optimally tuned
transactional tables.
</p><p>
In situations where integrity is of highest importance, MySQL
Server offers transaction-level reliability and integrity even
for non-transactional tables. If you lock tables with
<code class="literal">LOCK TABLES</code>, all updates stall until
integrity checks are made. If you obtain a <code class="literal">READ
LOCAL</code> lock (as opposed to a write lock) for a table
that allows concurrent inserts at the end of the table, reads
are allowed, as are inserts by other clients. The newly
inserted records are not be seen by the client that has the
read lock until it releases the lock. With <code class="literal">INSERT
DELAYED</code>, you can queue inserts into a local queue,
until the locks are released, without having the client wait
for the insert to complete. See
<a href="sql-syntax.html#insert-delayed" title="13.2.4.2. INSERT DELAYED Syntax">Section 13.2.4.2, “<code class="literal">INSERT DELAYED</code> Syntax”</a>.
</p><p>
“<span class="quote">Atomic,</span>” in the sense that we mean it, is
nothing magical. It only means that you can be sure that while
each specific update is running, no other user can interfere
with it, and there can never be an automatic rollback (which
can happen with transactional tables if you are not very
careful). MySQL Server also guarantees that there are no dirty
reads.
</p><p>
Following are some techniques for working with
non-transactional tables:
</p><div class="itemizedlist"><ul type="disc"><li><p>
Loops that need transactions normally can be coded with
the help of <code class="literal">LOCK TABLES</code>, and you don't
need cursors to update records on the fly.
</p></li><li><p>
To avoid using <code class="literal">ROLLBACK</code>, you can use
the following strategy:
</p><div class="orderedlist"><ol type="1"><li><p>
Use <code class="literal">LOCK TABLES</code> to lock all the
tables you want to access.
</p></li><li><p>
Test the conditions that must be true before
performing the update.
</p></li><li><p>
Update if everything is okay.
</p></li><li><p>
Use <code class="literal">UNLOCK TABLES</code> to release your
locks.
</p></li></ol></div><p>
This is usually a much faster method than using
transactions with possible rollbacks, although not always.
The only situation this solution doesn't handle is when
someone kills the threads in the middle of an update. In
this case, all locks are released but some of the updates
may not have been executed.
</p></li><li><p>
You can also use functions to update records in a single
operation. You can get a very efficient application by
using the following techniques:
</p><div class="itemizedlist"><ul type="circle"><li><p>
Modify columns relative to their current value.
</p></li><li><p>
Update only those columns that actually have changed.
</p></li></ul></div><p>
For example, when we are doing updates to some customer
information, we update only the customer data that has
changed and test only that none of the changed data, or
data that depends on the changed data, has changed
compared to the original row. The test for changed data is
done with the <code class="literal">WHERE</code> clause in the
<code class="literal">UPDATE</code> statement. If the record wasn't
updated, we give the client a message: “<span class="quote">Some of the
data you have changed has been changed by another
user.</span>” Then we show the old row versus the new row
in a window so that the user can decide which version of
the customer record to use.
</p><p>
This gives us something that is similar to column locking
but is actually even better because we only update some of
the columns, using values that are relative to their
current values. This means that typical
<code class="literal">UPDATE</code> statements look something like
these:
</p><pre class="programlisting">UPDATE tablename SET pay_back=pay_back+125;
UPDATE customer
SET
customer_date='current_date',
address='new address',
phone='new phone',
money_owed_to_us=money_owed_to_us-125
WHERE
customer_id=id AND address='old address' AND phone='old phone';
</pre><p>
This is very efficient and works even if another client
has changed the values in the <code class="literal">pay_back</code>
or <code class="literal">money_owed_to_us</code> columns.
</p></li><li><p>
<a class="indexterm" name="id2765755"></a>
<a class="indexterm" name="id2765764"></a>
In many cases, users have wanted <code class="literal">LOCK
TABLES</code> and/or <code class="literal">ROLLBACK</code> for
the purpose of managing unique identifiers. This can be
handled much more efficiently without locking or rolling
back by using an <code class="literal">AUTO_INCREMENT</code> column
and either the <code class="literal">LAST_INSERT_ID()</code> SQL
function or the <code class="literal">mysql_insert_id()</code> C API
function. See <a href="functions.html#information-functions" title="12.9.3. Information Functions">Section 12.9.3, “Information Functions”</a>. See
<a href="apis.html#mysql-insert-id" title="22.2.3.36. mysql_insert_id()">Section 22.2.3.36, “<code class="literal">mysql_insert_id()</code>”</a>.
</p><a class="indexterm" name="id2765809"></a><a class="indexterm" name="id2765819"></a><p>
You can generally code around the need for row-level
locking. Some situations really do need it, and
<code class="literal">InnoDB</code> tables support row-level
locking. With <code class="literal">MyISAM</code> tables, you can
use a flag column in the table and do something like the
following:
</p><pre class="programlisting">UPDATE <em class="replaceable"><code>tbl_name</code></em> SET row_flag=1 WHERE id=ID;
</pre><p>
MySQL returns <code class="literal">1</code> for the number of
affected rows if the row was found and
<code class="literal">row_flag</code> wasn't <code class="literal">1</code> in
the original row.
</p><p>
You can think of it as though MySQL Server changed the
preceding query to:
</p><pre class="programlisting">UPDATE <em class="replaceable"><code>tbl_name</code></em> SET row_flag=1 WHERE id=ID AND row_flag <> 1;
</pre></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="ansi-diff-triggers"></a>1.8.5.4. Stored Procedures and Triggers</h4></div></div></div><a class="indexterm" name="id2765897"></a><a class="indexterm" name="id2765908"></a><a class="indexterm" name="id2765918"></a><p>
Stored procedures are implemented in MySQL version 5.0. See
<a href="stored-procedures.html" title="Chapter 17. Stored Procedures and Functions">Chapter 17, <i>Stored Procedures and Functions</i></a>.
</p><p>
Basic triggers functionality is implemented in MySQL beginning
with version 5.0.2, with further development planned in MySQL
5.1. See <a href="triggers.html" title="Chapter 18. Triggers">Chapter 18, <i>Triggers</i></a>.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="ansi-diff-foreign-keys"></a>1.8.5.5. Foreign Keys</h4></div></div></div><a class="indexterm" name="id2765957"></a><a class="indexterm" name="id2765964"></a><p>
In MySQL Server 3.23.44 and up, the <code class="literal">InnoDB</code>
storage engine supports checking of foreign key constraints,
including <code class="literal">CASCADE</code>, <code class="literal">ON
DELETE</code>, and <code class="literal">ON UPDATE</code>. See
<a href="storage-engines.html#innodb-foreign-key-constraints" title="14.2.6.4. FOREIGN KEY Constraints">Section 14.2.6.4, “<code class="literal">FOREIGN KEY</code> Constraints”</a>.
</p><p>
For storage engines other than <code class="literal">InnoDB</code>,
MySQL Server parses the <code class="literal">FOREIGN KEY</code> syntax
in <code class="literal">CREATE TABLE</code> statements, but does not
use or store it. In the future, the implementation will be
extended to store this information in the table specification
file so that it may be retrieved by
<span><strong class="command">mysqldump</strong></span> and ODBC. At a later stage,
foreign key constraints will be implemented for
<code class="literal">MyISAM</code> tables as well.
</p><p>
Foreign key enforcement offers several benefits to database
developers:
</p><div class="itemizedlist"><ul type="disc"><li><p>
Assuming proper design of the relationships, foreign key
constraints make it more difficult for a programmer to
introduce an inconsistency into the database.
</p></li><li><p>
Centralized checking of constraints by the database server
makes it unnecessary to perform these checks on the
application side. This eliminates the possibility that
different applications may not all check the constraints
in the same way.
</p></li><li><p>
Using cascading updates and deletes can simplify the
application code.
</p></li><li><p>
Properly designed foreign key rules aid in documenting
relationships between tables.
</p></li></ul></div><p>
Do keep in mind that these benefits come at the cost of
additional overhead for the database server to perform the
necessary checks. Additional checking by the server affects
performance, which for some applications may be sufficiently
undesirable as to be avoided if possible. (Some major
commercial applications have coded the foreign-key logic at
the application level for this reason.)
</p><p>
MySQL gives database developers the choice of which approach
to use. If you don't need foreign keys and want to avoid the
overhead associated with enforcing referential integrity, you
can choose another table type instead, such as
<code class="literal">MyISAM</code>. (For example, the
<code class="literal">MyISAM</code> storage engine offers very fast
performance for applications that perform only
<code class="literal">INSERT</code> and <code class="literal">SELECT</code>
operations, because the inserts can be performed concurrently
with retrievals. See <a href="optimization.html#table-locking" title="7.3.2. Table Locking Issues">Section 7.3.2, “Table Locking Issues”</a>.)
</p><p>
If you choose not to take advantage of referential integrity
checks, keep the following considerations in mind:
</p><div class="itemizedlist"><ul type="disc"><li><p>
In the absence of server-side foreign key relationship
checking, the application itself must handle relationship
issues. For example, it must take care to insert rows into
tables in the proper order, and to avoid creating orphaned
child records. It must also be able to recover from errors
that occur in the middle of multiple-record insert
operations.
</p></li><li><p>
If <code class="literal">ON DELETE</code> is the only referential
integrity capability an application needs, note that as of
MySQL Server 4.0, you can use multiple-table
<code class="literal">DELETE</code> statements to delete rows from
many tables with a single statement. See
<a href="sql-syntax.html#delete" title="13.2.1. DELETE Syntax">Section 13.2.1, “<code class="literal">DELETE</code> Syntax”</a>.
</p></li><li><p>
A workaround for the lack of <code class="literal">ON DELETE</code>
is to add the appropriate <code class="literal">DELETE</code>
statement to your application when you delete records from
a table that has a foreign key. In practice, this is often
as quick as using foreign keys, and is more portable.
</p></li></ul></div><p>
Be aware that the use of foreign keys can in some instances
lead to problems:
</p><div class="itemizedlist"><ul type="disc"><li><p>
Foreign key support addresses many referential integrity
issues, but it is still necessary to design key
relationships carefully to avoid circular rules or
incorrect combinations of cascading deletes.
</p></li><li><p>
It is not uncommon for a DBA to create a topology of
relationships that makes it difficult to restore
individual tables from a backup. (MySQL alleviates this
difficulty by allowing you to temporarily disable foreign
key checks when reloading a table that depends on other
tables. See
<a href="storage-engines.html#innodb-foreign-key-constraints" title="14.2.6.4. FOREIGN KEY Constraints">Section 14.2.6.4, “<code class="literal">FOREIGN KEY</code> Constraints”</a>. As of
MySQL 4.1.1, <span><strong class="command">mysqldump</strong></span> generates dump
files that take advantage of this capability automatically
when reloaded.)
</p></li></ul></div><p>
Note that foreign keys in SQL are used to check and enforce
referential integrity, not to join tables. If you want to get
results from multiple tables from a <code class="literal">SELECT</code>
statement, you do this by performing a join between them:
</p><pre class="programlisting">SELECT * FROM t1, t2 WHERE t1.id = t2.id;
</pre><p>
See <a href="sql-syntax.html#join" title="13.2.7.1. JOIN Syntax">Section 13.2.7.1, “<code class="literal">JOIN</code> Syntax”</a>. See
<a href="tutorial.html#example-foreign-keys" title="3.6.6. Using Foreign Keys">Section 3.6.6, “Using Foreign Keys”</a>.
</p><p>
The <code class="literal">FOREIGN KEY</code> syntax without <code class="literal">ON
DELETE ...</code> is often used by ODBC applications to
produce automatic <code class="literal">WHERE</code> clauses.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="ansi-diff-views"></a>1.8.5.6. Views</h4></div></div></div><a class="indexterm" name="id2766292"></a><a class="indexterm" name="id2766298"></a><p>
Views (including updatable views) are implemented in the 5.0
version of MySQL Server. Views are available in binary
releases from 5.0.1 and up. See <a href="views.html" title="Chapter 19. Views">Chapter 19, <i>Views</i></a>.
</p><p>
Views are useful for allowing users to access a set of
relations (tables) as if it were a single table, and limiting
their access to just that. Views can also be used to restrict
access to rows (a subset of a particular table). For access
control to columns, you can also use the sophisticated
privilege system in MySQL Server. See
<a href="database-administration.html#privilege-system" title="5.7. The MySQL Access Privilege System">Section 5.7, “The MySQL Access Privilege System”</a>.
</p><p>
In designing an implementation of views, our ambitious goal,
as much as is possible within the confines of SQL, has been
full compliance with “<span class="quote">Codd's Rule #6</span>” for
relational database systems: “<span class="quote">All views that are
theoretically updatable, should in practice also be
updatable.</span>”
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="ansi-diff-comments"></a>1.8.5.7. '<code class="literal">--</code>' as the Start of a Comment</h4></div></div></div><a class="indexterm" name="id2766365"></a><a class="indexterm" name="id2766375"></a><p>
Some other SQL databases use ‘<code class="option">--</code>’
to start comments. MySQL Server uses
‘<code class="literal">#</code>’ as the start comment
character. You can also use the C comment style <code class="literal">/*
this is a comment */</code> with MySQL Server. See
<a href="language-structure.html#comments" title="9.5. Comment Syntax">Section 9.5, “Comment Syntax”</a>.
</p><p>
MySQL Server 3.23.3 and above support the
‘<code class="literal">--</code>’ comment style, provided
the comment is followed by a space (or by a control character
such as a newline). The requirement for a space is to prevent
problems with automatically generated SQL queries that have
used something like the following code, where we automatically
insert the value of the payment for
<code class="literal">!payment!</code>:
</p><pre class="programlisting">UPDATE account SET credit=credit-!payment!
</pre><p>
Think about what happens if the value of
<code class="literal">payment</code> is a negative value such as
<code class="literal">-1</code>:
</p><pre class="programlisting">UPDATE account SET credit=credit--1
</pre><p>
<code class="literal">credit--1</code> is a legal expression in SQL, but
if <code class="literal">--</code> is interpreted as the start of a
comment, part of the expression is discarded. The result is a
statement that has a completely different meaning than
intended:
</p><pre class="programlisting">UPDATE account SET credit=credit
</pre><p>
The statement produces no change in value at all! This
illustrates that allowing comments to start with
‘<code class="literal">--</code>’ can have serious
consequences.
</p><p>
Using our implementation of this method of commenting in MySQL
Server 3.23.3 and up, <code class="literal">credit--1</code> is actually
safe.
</p><p>
Another safe feature is that the <span><strong class="command">mysql</strong></span>
command-line client removes all lines that start with
‘<code class="literal">--</code>’.
</p><p>
The following information is relevant only if you are running
a MySQL version earlier than 3.23.3:
</p><p>
If you have an SQL program in a text file that contains
‘<code class="literal">--</code>’ comments, you should use
the <span><strong class="command">replace</strong></span> utility as follows to convert
the comments to use ‘<code class="literal">#</code>’
characters:
</p><pre class="programlisting">shell> <strong class="userinput"><code>replace " --" " #" < text-file-with-funny-comments.sql \</code></strong>
<strong class="userinput"><code>| mysql <em class="replaceable"><code>db_name</code></em></code></strong>
</pre><p>
instead of the usual:
</p><pre class="programlisting">shell> <strong class="userinput"><code>mysql <em class="replaceable"><code>db_name</code></em> < text-file-with-funny-comments.sql</code></strong>
</pre><p>
You can also edit the command file “<span class="quote">in place</span>” to
change the ‘<code class="literal">--</code>’ comments to
‘<code class="literal">#</code>’ comments:
</p><pre class="programlisting">shell> <strong class="userinput"><code>replace " --" " #" -- text-file-with-funny-comments.sql</code></strong>
</pre><p>
Change them back with this command:
</p><pre class="programlisting">shell> <strong class="userinput"><code>replace " #" " --" -- text-file-with-funny-comments.sql</code></strong>
</pre></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="constraints"></a>1.8.6. How MySQL Deals with Constraints</h3></div></div></div><div class="toc"><dl><dt><span class="section"><a href="introduction.html#constraint-primary-key">1.8.6.1. <code class="literal">PRIMARY KEY</code> and <code class="literal">UNIQUE</code> Index Constraints</a></span></dt><dt><span class="section"><a href="introduction.html#constraint-invalid-data">1.8.6.2. Constraints on Invalid Data</a></span></dt><dt><span class="section"><a href="introduction.html#constraint-enum">1.8.6.3. <code class="literal">ENUM</code> and <code class="literal">SET</code> Constraints</a></span></dt></dl></div><a class="indexterm" name="id2766613"></a><p>
MySQL allows you to work both with transactional tables that
allow rollback and with non-transactional tables that do not.
Because of this, constraint handling is a bit different in MySQL
than in other DBMSs. We must handle the case when you have
inserted or updated a lot of rows in a non-transactional table
for which changes cannot be rolled back when an error occurs.
</p><p>
The basic philosophy is that MySQL Server tries to produce an
error for anything that it can detect while parsing a statement
to be executed, and tries to recover from any errors that occur
while executing the statement. We do this in most cases, but not
yet for all.
</p><p>
The options MySQL has when an error occurs are to stop the
statement in the middle or to recover as well as possible from
the problem and continue. By default, the server follows the
latter course. This means, for example, that the server may
coerce illegal values to the closest legal values.
</p><p>
Beginning with MySQL 5.0.2, several SQL mode options are
available to provide greater control over how accepting to be of
bad data values and whether to continue executing a statement or
abort it when errors occur. Using these options, you can
configure MySQL Server to act in a more traditional fashion that
is like other DBMSs that reject improper input. The SQL mode can
be set at runtime, which enables individual clients to select
the behavior most appropriate for their requirements. See
<a href="database-administration.html#server-sql-mode" title="5.3.2. The Server SQL Mode">Section 5.3.2, “The Server SQL Mode”</a>.
</p><p>
The following sections describe what happens for the different
types of constraints.
</p><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="constraint-primary-key"></a>1.8.6.1. <code class="literal">PRIMARY KEY</code> and <code class="literal">UNIQUE</code> Index Constraints</h4></div></div></div><a class="indexterm" name="id2756094"></a><a class="indexterm" name="id2756104"></a><a class="indexterm" name="id2756114"></a><p>
Normally, an error occurs when you try to
<code class="literal">INSERT</code> or <code class="literal">UPDATE</code> a row
that causes a primary key, unique key, or foreign key
violation. If you are using a transactional storage engine
such as <code class="literal">InnoDB</code>, MySQL automatically rolls
back the statement. If you are using a non-transactional
storage engine, MySQL stops processing the statement at the
row for which the error occurred and leaves any remaining rows
unprocessed.
</p><p>
If you wish to ignore such key violations, MySQL supports an
<code class="literal">IGNORE</code> keyword for
<code class="literal">INSERT</code> and <code class="literal">UPDATE</code>. In
this case, MySQL ignores any key violations and continues
processing with the next row. See <a href="sql-syntax.html#insert" title="13.2.4. INSERT Syntax">Section 13.2.4, “<code class="literal">INSERT</code> Syntax”</a>.
See <a href="sql-syntax.html#update" title="13.2.10. UPDATE Syntax">Section 13.2.10, “<code class="literal">UPDATE</code> Syntax”</a>.
</p><p>
You can get information about the number of rows actually
inserted or updated with the <code class="literal">mysql_info()</code> C
API function. See <a href="apis.html#mysql-info" title="22.2.3.34. mysql_info()">Section 22.2.3.34, “<code class="literal">mysql_info()</code>”</a>. In MySQL 4.1
and up, you also can use the <code class="literal">SHOW WARNINGS</code>
statement. See <a href="sql-syntax.html#show-warnings" title="13.5.4.22. SHOW WARNINGS Syntax">Section 13.5.4.22, “<code class="literal">SHOW WARNINGS</code> Syntax”</a>.
</p><p>
For the moment, only <code class="literal">InnoDB</code> tables support
foreign keys. See
<a href="storage-engines.html#innodb-foreign-key-constraints" title="14.2.6.4. FOREIGN KEY Constraints">Section 14.2.6.4, “<code class="literal">FOREIGN KEY</code> Constraints”</a>. Foreign key
support in <code class="literal">MyISAM</code> tables is scheduled for
implementation in MySQL 5.1.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="constraint-invalid-data"></a>1.8.6.2. Constraints on Invalid Data</h4></div></div></div><a class="indexterm" name="id2766968"></a><a class="indexterm" name="id2766978"></a><a class="indexterm" name="id2766988"></a><a class="indexterm" name="id2766998"></a><a class="indexterm" name="id2767008"></a><p>
Before MySQL 5.0.2, MySQL is forgiving of illegal or improper
data values and coerces them to legal values for data entry.
In MySQL 5.0.2 and up, that remains the default behavior, but
you can select more traditional treatment of bad values such
that the server rejects them and aborts the statement in which
they occur. This section describes the default (forgiving)
behavior of MySQL, as well as the newer strict SQL mode and
how it differs.
</p><p>
The following holds true when you are not using strict mode.
If you insert an “<span class="quote">incorrect</span>” value into a column,
such as a <code class="literal">NULL</code> into a <code class="literal">NOT
NULL</code> column or a too-large numeric value into a
numeric column, MySQL sets the column to the “<span class="quote">best
possible value</span>” instead of producing an error:
</p><div class="itemizedlist"><ul type="disc"><li><p>
If you try to store an out of range value in a numeric
column, MySQL Server instead stores zero, the smallest
possible value, or the largest possible value in the
column.
</p></li><li><p>
For strings, MySQL stores either the empty string or as
much of the string as can be stored in the column.
</p></li><li><p>
If you try to store a string that doesn't start with a
number into a numeric column, MySQL Server stores 0.
</p></li><li><p>
MySQL allows you to store certain incorrect date values
into <code class="literal">DATE</code> and
<code class="literal">DATETIME</code> columns (such as
<code class="literal">'2000-02-31'</code> or
<code class="literal">'2000-02-00'</code>). The idea is that it's
not the job of the SQL server to validate dates. If MySQL
can store a date value and retrieve exactly the same
value, MySQL stores it as given. If the date is totally
wrong (outside the server's ability to store it), the
special date value <code class="literal">'0000-00-00'</code> is
stored in the column instead.
</p></li><li><p>
If you try to store <code class="literal">NULL</code> into a column
that doesn't take <code class="literal">NULL</code> values, an error
occurs for single-row <code class="literal">INSERT</code>
statements. For multiple-row <code class="literal">INSERT</code>
statements or for <code class="literal">INSERT INTO ...
SELECT</code> statements, MySQL Server stores the
implicit default value for the column data type. In
general, this is <code class="literal">0</code> for numeric types,
the empty string (<code class="literal">''</code>) for string types,
and the “<span class="quote">zero</span>” value for date and time types.
Implicit default values are discussed in
<a href="sql-syntax.html#create-table" title="13.1.5. CREATE TABLE Syntax">Section 13.1.5, “<code class="literal">CREATE TABLE</code> Syntax”</a>.
</p></li><li><p>
If an <code class="literal">INSERT</code> statement specifies no
value for a column, MySQL inserts its default value if the
column definition includes an explicit
<code class="literal">DEFAULT</code> clause. If the definition has
no such <code class="literal">DEFAULT</code> clause, MySQL inserts
the implicit default value for the column data type.
</p></li></ul></div><p>
The reason for the preceding rules is that we can't check
these conditions until the statement has begun executing. We
can't just roll back if we encounter a problem after updating
a few rows, because the storage engine may not support
rollback. The option of terminating the statement is not that
good; in this case, the update would be “<span class="quote">half
done,</span>” which is probably the worst possible scenario.
In this case, it's better to “<span class="quote">do the best you
can</span>” and then continue as if nothing happened.
</p><p>
In MySQL 5.0.2 and up, you can select stricter treatment of
input values by using the
<code class="literal">STRICT_TRANS_TABLES</code> or
<code class="literal">STRICT_ALL_TABLES</code> SQL modes. See
<a href="database-administration.html#server-sql-mode" title="5.3.2. The Server SQL Mode">Section 5.3.2, “The Server SQL Mode”</a>.
</p><p>
<code class="literal">STRICT_TRANS_TABLES</code> works like this:
</p><div class="itemizedlist"><ul type="disc"><li><p>
For transactional storage engines, bad data values
occurring anywhere in the statement cause the statement to
abort and roll back.
</p></li><li><p>
For non-transactional storage engines, the statement
aborts if the error occurs in the first row to be inserted
or updated. (In this case, the statement can be regarded
to leave the table unchanged, just as for a transactional
table.) Errors in rows after the first do not abort the
statement. Instead, bad data values are adjusted and
result in warnings rather than errors. In other words,
with <code class="literal">STRICT_TRANS_TABLES</code>, a wrong value
causes MySQL to roll back, if it can, all updates done so
far.
</p></li></ul></div><p>
For stricter checking, enable
<code class="literal">STRICT_ALL_TABLES</code>. This is the same as
<code class="literal">STRICT_TRANS_TABLES</code> except that for
non-transactional storage engines, errors abort the statement
even for bad data in rows following the first row. This means
that if an error occurs partway through a multiple-row insert
or update for a non-transactional table, a partial update
results. Earlier rows are inserted or updated, but those from
the point of the error on are not. To avoid this for
non-transactional tables, either use single-row statements or
else use <code class="literal">STRICT_TRANS_TABLES</code> if conversion
warnings rather than errors are acceptable. To avoid problems
in the first place, do not use MySQL to check column content.
It is safest (and often faster) to let the application ensure
that it passes only legal values to the database.
</p><p>
With either of the strict mode options, you can cause errors
to be treated as warnings by using <code class="literal">INSERT
IGNORE</code> or <code class="literal">UPDATE IGNORE</code> rather
than <code class="literal">INSERT</code> or <code class="literal">UPDATE</code>
without <code class="literal">IGNORE</code>.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="constraint-enum"></a>1.8.6.3. <code class="literal">ENUM</code> and <code class="literal">SET</code> Constraints</h4></div></div></div><p>
<code class="literal">ENUM</code> and <code class="literal">SET</code> columns
provide an efficient way to define columns that can contain
only a given set of values. However, before MySQL 5.0.2,
<code class="literal">ENUM</code> and <code class="literal">SET</code> are not
real constraints. This is for the same reasons that
<code class="literal">NOT NULL</code> is not honored. See
<a href="introduction.html#constraint-invalid-data" title="1.8.6.2. Constraints on Invalid Data">Section 1.8.6.2, “Constraints on Invalid Data”</a>.
</p><p>
<code class="literal">ENUM</code> columns always have a default value.
If you don't specify a default value, then it is
<code class="literal">NULL</code> for columns that can have
<code class="literal">NULL</code>, otherwise the first enumeration value
is used as the default value.
</p><p>
If you insert an incorrect value into an
<code class="literal">ENUM</code> column or if you force a value into an
<code class="literal">ENUM</code> column with <code class="literal">IGNORE</code>,
it is set to the reserved enumeration value of
<code class="literal">0</code>, which is displayed as an empty string in
string context. See <a href="column-types.html#enum" title="11.4.4. The ENUM Type">Section 11.4.4, “The <code class="literal">ENUM</code> Type”</a>.
</p><p>
If you insert an incorrect value into a <code class="literal">SET</code>
column, the incorrect value is ignored. For example, if the
column can contain the values <code class="literal">'a'</code>,
<code class="literal">'b'</code>, and <code class="literal">'c'</code>, an attempt
to assign <code class="literal">'a,x,b,y'</code> results in a value of
<code class="literal">'a,b'</code>. See <a href="column-types.html#set" title="11.4.5. The SET Type">Section 11.4.5, “The <code class="literal">SET</code> Type”</a>.
</p><p>
As of MySQL 5.0.2, you can configure the server to use strict
SQL mode. See <a href="database-administration.html#server-sql-mode" title="5.3.2. The Server SQL Mode">Section 5.3.2, “The Server SQL Mode”</a>. When strict
mode is enabled, the definition of a <code class="literal">ENUM</code>
or <code class="literal">SET</code> column does act as a constraint on
values entered into the column. An error occurs for values
that do not satisfy these conditions:
</p><div class="itemizedlist"><ul type="disc"><li><p>
An <code class="literal">ENUM</code> value must be one of those
listed in the column definition, or the internal numeric
equivalent thereof. The value cannot be the error value
(that is, 0 or the empty string). For a column defined as
<code class="literal">ENUM('a','b','c')</code>, values such as
<code class="literal">''</code>, <code class="literal">'d'</code>, and
<code class="literal">'ax'</code> are illegal and are rejected.
</p></li><li><p>
A <code class="literal">SET</code> value must be the empty string or
a value consisting of one or more of the values listed in
the column definition separated by commas. For a column
defined as <code class="literal">SET('a','b','c')</code>, values
such as <code class="literal">'d'</code>, and
<code class="literal">'a,b,c,d'</code> are illegal and are rejected.
</p></li></ul></div><p>
Errors for invalid values can be suppressed in strict mode if
you use <code class="literal">INSERT IGNORE</code> or <code class="literal">UPDATE
IGNORE</code>. In this case, a warning is generated rather
than an error. For <code class="literal">ENUM</code>, the value is
inserted as the error member (<code class="literal">0</code>). For
<code class="literal">SET</code>, the value is inserted as given except
that any invalid substrings are deleted. For example,
<code class="literal">'a,x,b,y'</code> results in a value of
<code class="literal">'a,b'</code>, as described earlier.
</p></div></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="preface.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="installing.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Preface </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 2. Installing MySQL</td></tr></table></div></body></html>
