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peterepetere
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Description of table expressions, including join syntax, from Robert B.
Easter <reaster@comptechnews.com>, heavily massaged by me. Also cleaned up value expressions a bit.
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‎doc/src/sgml/func.sgml

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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.48 2001/01/20 20:59:28 petere Exp $ -->
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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.49 2001/01/21 22:02:01 petere Exp $ -->
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<chapter id="functions">
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<title>Functions and Operators</title>
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other implementations.
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</para>
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<sect1 id="functions-logical">
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<title>Logical Operators</title>
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<para>
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The usual logical operators are available:
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<simplelist>
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<member>AND</member>
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<member>OR</member>
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<member>NOT</member>
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</simplelist>
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SQL uses a three-valued boolean logic where NULL represents
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<quote>unknown</quote>. Observe the following truth tables:
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<informaltable>
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<tgroup cols="4">
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<thead>
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<row>
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<entry><replaceable>a</replaceable></entry>
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<entry><replaceable>b</replaceable></entry>
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<entry><replaceable>a</replaceable> AND <replaceable>b</replaceable></entry>
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<entry><replaceable>a</replaceable> OR <replaceable>b</replaceable></entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>TRUE</entry>
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<entry>TRUE</entry>
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<entry>TRUE</entry>
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<entry>TRUE</entry>
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</row>
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<row>
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<entry>TRUE</entry>
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<entry>FALSE</entry>
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<entry>FALSE</entry>
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<entry>TRUE</entry>
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</row>
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<row>
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<entry>TRUE</entry>
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<entry>NULL</entry>
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<entry>NULL</entry>
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<entry>TRUE</entry>
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</row>
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<row>
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<entry>FALSE</entry>
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<entry>FALSE</entry>
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<entry>FALSE</entry>
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<entry>FALSE</entry>
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</row>
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<row>
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<entry>FALSE</entry>
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<entry>NULL</entry>
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<entry>FALSE</entry>
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<entry>NULL</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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<informaltable>
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<tgroup cols="2">
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<thead>
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<row>
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<entry><replaceable>a</replaceable></entry>
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<entry>NOT <replaceable>a</replaceable></entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>TRUE</entry>
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<entry>FALSE</entry>
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</row>
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<row>
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<entry>FALSE</entry>
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<entry>TRUE</entry>
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</row>
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<row>
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<entry>NULL</entry>
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<entry>NULL</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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</para>
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</sect1>
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<sect1 id="functions-comparison">
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<title>Comparison Operators</title>
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<literal>&lt;</literal> operator to compare a boolean with
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<literal>3</literal>).
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</para>
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<para>
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To check whether a value is or is not NULL, use the constructs
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<synopsis>
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<replaceable>expression</replaceable> IS NULL
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<replaceable>expression</replaceable> IS NOT NULL
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</synopsis>
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Do <emphasis>not</emphasis> use
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<literal><replaceable>expression</replaceable> = NULL</literal>
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because NULL is not <quote>equal to</quote> NULL. (NULL represents
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an unknown value, so it is not known whether two unknown values are
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equal.) <productname>Postgres</productname> implicitly converts
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<literal>= NULL</literal> clauses to <literal>IS NULL</literal> to
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allow some broken client applications (such as
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<productname>Microsoft Access</productname>) to work, but this may
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be discontinued in a future release.
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</para>
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</sect1>
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