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<H1><A NAME="SECTION00820000000000000000"> </A>
<A NAME="structure-of-tables"> </A>
<BR>
Structure of Tables
</H1>
<P>
Table data are arranged in columns and rows, and stored in MIDAS files with the extension
<TT>.tbl</TT>. The entry at a given row and column may be either a single value
or an array.
The items in one row may describe different properties of the same
object or feature. All elements in a given column must be
of the <I>same</I> type and thus be associated with the <I>same</I> property. For instance, a table
with stellar data could contain the following items in each row:
identification, right ascension, declination, magnitude, and spectral type.
The first column would then contain all the stellar identifications, the
second the right ascensions, etc.
<P>
The supported column data types are numerical data (8/16/32-bit integers or 32/64-bit
reals)
and character strings.
<P>
Each column is tagged with a user-defined label, a display format and optional physical
units and can be referred to either by its absolute number or its label.
<P>
An item in a table is accessed by giving its column and row in addition to the
table name. The row number can either be given as an absolute value (i.e. the
sequence number) or indicated by the value in a previously defined reference
column.
<P>
In addition to the normal columns all tables contain a <TT>SELECT</TT> and a <TT>SEQUENCE</TT> column:
<UL>
<LI>The <TT>SELECT</TT> column enables the user to define and work with
a subset of his table by flagging the rows that satisfy a selection
criteria. The subtable will be used by commands that do not modify the
table information whereas the selection flag will be reset by commands
that modify table information and this before taking any action.
The values of this column can be accessed in the <TT>COMPUTE</TT> command
by using the name SELECT (short form SEL).
<LI>The <TT>SEQUENCE</TT> column contains the sequence number of each
row. The values of this column can be accessed in the <TT>COMPUTE</TT> command
by using the name SEQUENCE (short form SEQ).
</UL>
<P>
If an element is not defined it will be a <TT>NULL</TT> entry and will be listed
as a "*" for all data types except for character strings. In that case it
will be listed as an empty field.
<P>
The tables may be physically stored on disk in two formats: by records
corresponding to the natural way of storing sequentially the rows and transposed
, where all the values of a given column are stored together( default mode).
A table can be always expanded in the sense that its number of columns and rows
is automatically increased when the allocated space is exceeded.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-09</I>
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