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<h1 class="pudge-member-page-heading">SelectResults: Using Queries</h1>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents:</p>
<ul class="simple">
<li><a href="#overview" class="reference internal" id="id1">Overview</a></li>
<li><a href="#retrieval-methods" class="reference internal" id="id2">Retrieval Methods</a><ul>
<li><a href="#iteration" class="reference internal" id="id3">Iteration</a></li>
<li><a href="#getone-default-optional" class="reference internal" id="id4"><tt class="docutils literal">getOne(default=optional)</tt></a></li>
</ul>
</li>
<li><a href="#cloning-methods" class="reference internal" id="id5">Cloning Methods</a><ul>
<li><a href="#orderby-column" class="reference internal" id="id6"><tt class="docutils literal">orderBy(column)</tt></a></li>
<li><a href="#limit-num" class="reference internal" id="id7"><tt class="docutils literal">limit(num)</tt></a></li>
<li><a href="#lazycolumns-v" class="reference internal" id="id8"><tt class="docutils literal">lazyColumns(v)</tt></a></li>
<li><a href="#reversed" class="reference internal" id="id9"><tt class="docutils literal">reversed()</tt></a></li>
<li><a href="#distinct" class="reference internal" id="id10"><tt class="docutils literal">distinct()</tt></a></li>
<li><a href="#filter-expression" class="reference internal" id="id11"><tt class="docutils literal">filter(expression)</tt></a></li>
</ul>
</li>
<li><a href="#aggregate-methods" class="reference internal" id="id12">Aggregate Methods</a><ul>
<li><a href="#count" class="reference internal" id="id13"><tt class="docutils literal">count()</tt></a></li>
<li><a href="#sum-column" class="reference internal" id="id14"><tt class="docutils literal">sum(column)</tt></a></li>
<li><a href="#min-column" class="reference internal" id="id15"><tt class="docutils literal">min(column)</tt></a></li>
<li><a href="#max-column" class="reference internal" id="id16"><tt class="docutils literal">max(column)</tt></a></li>
<li><a href="#avg-column" class="reference internal" id="id17"><tt class="docutils literal">avg(column)</tt></a></li>
</ul>
</li>
<li><a href="#traversal-to-related-sqlobject-classes" class="reference internal" id="id18">Traversal to related SQLObject classes</a><ul>
<li><a href="#throughto-join-name-and-throughto-foreign-key-name" class="reference internal" id="id19"><tt class="docutils literal">throughTo.join_name and throughTo.foreign_key_name</tt></a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="overview">
<h1><a href="#id1" class="toc-backref">Overview</a></h1>
<p>SelectResults are returned from <tt class="docutils literal">.select</tt> and <tt class="docutils literal">.selectBy</tt> methods on SQLObject classes, and from <tt class="docutils literal">SQLMultipleJoin</tt>, and <tt class="docutils literal">SQLRelatedJoin</tt> accessors on SQLObject instances.</p>
<p>Select results are generators, which are lazily evaluated. The SQL
is only executed when you iterate over the select results, fetching
rows one at a time. This way you
can iterate over large results without keeping the entire result set
in memory. You can also do things like <tt class="docutils literal">.reversed()</tt> without
fetching and reversing the entire result -- instead, SQLObject can
change the SQL that is sent so you get equivalent results.</p>
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">To retrieve the results all at once use the python idiom
of calling <tt class="docutils literal">list()</tt> on the generator to force execution
and convert the results to a stored list.</p>
</div>
<p>You can also slice select results. This modifies the SQL query, so
<tt class="docutils literal"><span class="pre">peeps[:10]</span></tt> will result in <tt class="docutils literal">LIMIT 10</tt> being added to the end of
the SQL query. If the slice cannot be performed in the SQL (e.g.,
peeps[:-10]), then the select is executed, and the slice is performed
on the list of results. This will generally only happen when you use
negative indexes.</p>
<p>In certain cases, you may get a select result with an object in it
more than once, e.g., in some joins. If you don't want this, you can
add the keyword argument <tt class="docutils literal"><span class="pre">MyClass.select(...,</span> distinct=True)</tt>, which
results in a <tt class="docutils literal">SELECT DISTINCT</tt> call.</p>
<p>You can get the length of the result without fetching all the results
by calling <tt class="docutils literal">count</tt> on the result object, like
<tt class="docutils literal"><span class="pre">MyClass.select().count()</span></tt>. A <tt class="docutils literal"><span class="pre">COUNT(*)</span></tt> query is used -- the
actual objects are not fetched from the database. Together with
slicing, this makes batched queries easy to write:</p>
<pre class="literal-block">
start = 20
size = 10
query = Table.select()
results = query[start:start+size]
total = query.count()
print "Showing page %i of %i" % (start/size + 1, total/size + 1)
</pre>
<div class="note">
<p class="first admonition-title">Note</p>
<p>There are several factors when considering the efficiency of this
kind of batching, and it depends very much how the batching is
being used. Consider a web application where you are showing an
average of 100 results, 10 at a time, and the results are ordered
by the date they were added to the database. While slicing will
keep the database from returning all the results (and so save some
communication time), the database will still have to scan through
the entire result set to sort the items (so it knows which the
first ten are), and depending on your query may need to scan
through the entire table (depending on your use of indexes).
Indexes are probably the most important way to improve importance
in a case like this, and you may find caching to be more effective
than slicing.</p>
<p class="last">In this case, caching would mean retrieving the <em>complete</em> results.
You can use <tt class="docutils literal"><span class="pre">list(MyClass.select(...))</span></tt> to do this. You can save
these results for some limited period of time, as the user looks
through the results page by page. This means the first page in a
search result will be slightly more expensive, but all later pages
will be very cheap.</p>
</div>
</div>
<div class="section" id="retrieval-methods">
<h1><a href="#id2" class="toc-backref">Retrieval Methods</a></h1>
<div class="section" id="iteration">
<h2><a href="#id3" class="toc-backref">Iteration</a></h2>
<p>As mentioned in the overview, the typical way to access the results
is by treating it as a generator and iterating over it (in a loop,
by converting to a list, etc).</p>
</div>
<div class="section" id="getone-default-optional">
<h2><a href="#id4" class="toc-backref"><tt class="docutils literal">getOne(default=optional)</tt></a></h2>
<p>In cases where your restrictions cause there to always be a single record
in the result set, this method will return it or raise an exception:
SQLObjectIntegrityError if more than one result is found, or
SQLObjectNotFound if there are actually no results, unless you pass in
a default like <tt class="docutils literal">.getOne(None)</tt>.</p>
</div>
</div>
<div class="section" id="cloning-methods">
<h1><a href="#id5" class="toc-backref">Cloning Methods</a></h1>
<p>These methods return a modified copy of the SelectResult instance
they are called on, so successive calls can chained, eg
<tt class="docutils literal">results = <span class="pre">MyClass.selectBy(city='Boston').filter(MyClass.q.commute_distance>10).orderBy('vehicle_mileage')</span></tt>
or used independently later on.</p>
<div class="section" id="orderby-column">
<h2><a href="#id6" class="toc-backref"><tt class="docutils literal">orderBy(column)</tt></a></h2>
<p>Takes a string column name (optionally prefixed with '-' for DESCending)
or a <a href="SQLBuilder.html" class="reference external">SQLBuilder expression</a>.</p>
</div>
<div class="section" id="limit-num">
<h2><a href="#id7" class="toc-backref"><tt class="docutils literal">limit(num)</tt></a></h2>
<p>Only return first num many results. Equivalent to results[:num] slicing.</p>
</div>
<div class="section" id="lazycolumns-v">
<h2><a href="#id8" class="toc-backref"><tt class="docutils literal">lazyColumns(v)</tt></a></h2>
<p>Only fetch the IDs for the results, the rest of the columns will be
retrieved when attributes of the returned instances are accessed.</p>
</div>
<div class="section" id="reversed">
<h2><a href="#id9" class="toc-backref"><tt class="docutils literal">reversed()</tt></a></h2>
<p>Reverse-order. Alternative to calling orderBy with SQLBuilder.DESC or '-'.</p>
</div>
<div class="section" id="distinct">
<h2><a href="#id10" class="toc-backref"><tt class="docutils literal">distinct()</tt></a></h2>
<p>In SQL, SELECT DISTINCT, removing duplicate rows.</p>
</div>
<div class="section" id="filter-expression">
<h2><a href="#id11" class="toc-backref"><tt class="docutils literal">filter(expression)</tt></a></h2>
<p>Add additional expressions to restrict result set.
Takes either a string static SQL expression valid in a WHERE clause,
or a <a href="SQLBuilder.html" class="reference external">SQLBuilder expression</a>. ANDed with any previous expressions.</p>
</div>
</div>
<div class="section" id="aggregate-methods">
<h1><a href="#id12" class="toc-backref">Aggregate Methods</a></h1>
<p>These return column values (strings, numbers, etc)
not new SQLResults instances, by making the appropriate
SQL query (the actual result rows are not retrieved).
Any that take a column can also take a SQLBuilder
column instance, e.g. <tt class="docutils literal">MyClass.q.size</tt>.</p>
<div class="section" id="count">
<h2><a href="#id13" class="toc-backref"><tt class="docutils literal">count()</tt></a></h2>
<p>Returns the length of the result set, by a SQL <tt class="docutils literal">SELECT <span class="pre">COUNT(...)</span></tt>
query.</p>
</div>
<div class="section" id="sum-column">
<h2><a href="#id14" class="toc-backref"><tt class="docutils literal">sum(column)</tt></a></h2>
<p>The sum of values for <tt class="docutils literal">column</tt> in the result set.</p>
</div>
<div class="section" id="min-column">
<h2><a href="#id15" class="toc-backref"><tt class="docutils literal">min(column)</tt></a></h2>
<p>The minimum value for <tt class="docutils literal">column</tt> in the result set.</p>
</div>
<div class="section" id="max-column">
<h2><a href="#id16" class="toc-backref"><tt class="docutils literal">max(column)</tt></a></h2>
<p>The maximum value for <tt class="docutils literal">column</tt> in the result set.</p>
</div>
<div class="section" id="avg-column">
<h2><a href="#id17" class="toc-backref"><tt class="docutils literal">avg(column)</tt></a></h2>
<p>The average value for the <tt class="docutils literal">column</tt> in the result set.</p>
</div>
</div>
<div class="section" id="traversal-to-related-sqlobject-classes">
<h1><a href="#id18" class="toc-backref">Traversal to related SQLObject classes</a></h1>
<div class="section" id="throughto-join-name-and-throughto-foreign-key-name">
<h2><a href="#id19" class="toc-backref"><tt class="docutils literal">throughTo.join_name and throughTo.foreign_key_name</tt></a></h2>
<p>This accessor lets you retrieve the objects related to your
SelectResults by either a join or foreign key relationship,
in the same manner as the cloning methods above. For instance:</p>
<pre class="literal-block">
Schools.select(Schools.q.student_satisfaction>90).throughTo.teachers
</pre>
<p>returns a SelectResult of Teachers of Schools with satisfied students,
assuming Schools has a SQLMultipleJoin or SQLRelatedJoin attribute
named <tt class="docutils literal">teachers</tt>. Similarily, with a self-joining foreign key named
<tt class="docutils literal">father</tt>:</p>
<pre class="literal-block">
Person.select(Person.q.name=='Steve').throughTo.father.throughTo.father
</pre>
<p>returns a SelectResult of Persons who are the paternal grandfather of someone
named <tt class="docutils literal">Steve</tt>.</p>
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