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<chapter id="functions">

<title>Functions and Operators</title>

<citetitle>Programmer's Guide</citetitle>. The

<application>psql</application> commands <command>\df</command> and

<command>\do</command> can be used to show the list of all actually

availablefunction and operators, respectively.

availablefunctions and operators, respectively.

</para>

<para>

<entry>FALSE</entry>

<entry>NULL</entry>

</row>

<row>

<entry>NULL</entry>

<entry>NULL</entry>

<entry>NULL</entry>

<entry>NULL</entry>

</row>

</tbody>

</tgroup>

</informaltable>

<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

equal.) <productname>Postgres</productname>presently converts

<literal>x= NULL</literal> clauses to <literal>xIS 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.

<row>

<entry> <literal>!!</literal> </entry>

<entry>Factorial (left operator)</entry>

<entry>Factorial (prefix operator)</entry>

<entry>!! 5</entry>

<entry>120</entry>

</row>