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Description of table expressions, including join syntax, from Robert B.

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Easter <reaster@comptechnews.com>, heavily massaged by me.  Also cleaned up
value expressions a bit.
This commit is contained in:
Peter Eisentraut 2001-01-21 22:02:01 +00:00
parent d7d51bc138
commit 4f34f55d3d
2 changed files with 848 additions and 223 deletions
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115
doc/src/sgml/func.sgml
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@ -1,4 +1,4 @@
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.48 2001/01/20 20:59:28 petere Exp $ -->
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.49 2001/01/21 22:02:01 petere Exp $ -->
<chapter id="functions">
<title>Functions and Operators</title>
@ -24,6 +24,102 @@
other implementations.
</para>
<sect1 id="functions-logical">
<title>Logical Operators</title>
<para>
The usual logical operators are available:
<simplelist>
<member>AND</member>
<member>OR</member>
<member>NOT</member>
</simplelist>
SQL uses a three-valued boolean logic where NULL represents
<quote>unknown</quote>. Observe the following truth tables:
<informaltable>
<tgroup cols="4">
<thead>
<row>
<entry><replaceable>a</replaceable></entry>
<entry><replaceable>b</replaceable></entry>
<entry><replaceable>a</replaceable> AND <replaceable>b</replaceable></entry>
<entry><replaceable>a</replaceable> OR <replaceable>b</replaceable></entry>
</row>
</thead>
<tbody>
<row>
<entry>TRUE</entry>
<entry>TRUE</entry>
<entry>TRUE</entry>
<entry>TRUE</entry>
</row>
<row>
<entry>TRUE</entry>
<entry>FALSE</entry>
<entry>FALSE</entry>
<entry>TRUE</entry>
</row>
<row>
<entry>TRUE</entry>
<entry>NULL</entry>
<entry>NULL</entry>
<entry>TRUE</entry>
</row>
<row>
<entry>FALSE</entry>
<entry>FALSE</entry>
<entry>FALSE</entry>
<entry>FALSE</entry>
</row>
<row>
<entry>FALSE</entry>
<entry>NULL</entry>
<entry>FALSE</entry>
<entry>NULL</entry>
</row>
</tbody>
</tgroup>
</informaltable>
<informaltable>
<tgroup cols="2">
<thead>
<row>
<entry><replaceable>a</replaceable></entry>
<entry>NOT <replaceable>a</replaceable></entry>
</row>
</thead>
<tbody>
<row>
<entry>TRUE</entry>
<entry>FALSE</entry>
</row>
<row>
<entry>FALSE</entry>
<entry>TRUE</entry>
</row>
<row>
<entry>NULL</entry>
<entry>NULL</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect1>
<sect1 id="functions-comparison">
<title>Comparison Operators</title>
@ -88,6 +184,23 @@
<literal>&lt;</literal> operator to compare a boolean with
<literal>3</literal>).
</para>
<para>
To check whether a value is or is not NULL, use the constructs
<synopsis>
<replaceable>expression</replaceable> IS NULL
<replaceable>expression</replaceable> IS NOT NULL
</synopsis>
Do <emphasis>not</emphasis> use
<literal><replaceable>expression</replaceable> = NULL</literal>
because NULL is not <quote>equal to</quote> NULL. (NULL represents
an unknown value, so it is not known whether two unknown values are
equal.) <productname>Postgres</productname> implicitly converts
<literal>= NULL</literal> clauses to <literal>IS NULL</literal> to
allow some broken client applications (such as
<productname>Microsoft Access</productname>) to work, but this may
be discontinued in a future release.
</para>
</sect1>

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doc/src/sgml/syntax.sgml
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