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Editorialization of some text discussing 'IS NULL' vs. '= NULL'.

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This commit is contained in:
Neil Conway 2004-03-07 01:01:44 +00:00
parent 565b4f2de5
commit ab8faed00d
1 changed files with 12 additions and 10 deletions
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22
doc/src/sgml/func.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.191 2004/03/05 02:41:14 momjian Exp $
$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.192 2004/03/07 01:01:44 neilc Exp $
PostgreSQL documentation
-->
@ -301,20 +301,22 @@ PostgreSQL documentation
<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 may expect 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>