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$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.192 2004/03/07 01:01:44 neilc Exp $

PostgreSQL documentation

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<literal><replaceable>expression</replaceable> = NULL</literal>

because <literal>NULL</> is not <quote>equal to</quote>

<literal>NULL</>. (The null value represents an unknown value,

and it is not known whether two unknown values are equal.)

and it is not known whether two unknown values are equal.) This

behavior conforms to the SQL standard.

</para>

<para>

Some applications may(incorrectly) require that

Some applications mayexpect that

<literal><replaceable>expression</replaceable> = NULL</literal>

returns true if <replaceable>expression</replaceable> evaluates to

the null value. To support these applications, the run-time option

<varname>transform_null_equals</varname> can be turned on (e.g.,

<literal>SET transform_null_equals TO ON;</literal>).

<productname>PostgreSQL</productname> will then convert

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

<literal>x IS NULL</literal>. This was

the default behavior in releases 6.5 through 7.1.

the null value. It is highly recommended that these applications

be modified to comply with the SQL standard. However, if that

cannot be done the <varname>transform_null_equals</varname>

configuration variable is available. If it is enabled,

<productname>PostgreSQL</productname> will convert <literal>x =

NULL</literal> clauses to <literal>x IS NULL</literal>. This was

the default behavior in <productname>PostgreSQL</productname>

releases 6.5 through 7.1.

</para>

<para>