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Improve documentation for COALESCE and NULLIF. Add references for NVL

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and IFNULL.

Backpatch to 8.1.X.
This commit is contained in:
Bruce Momjian 2005-11-28 23:18:48 +00:00
parent 200545039c
commit ee4aa3021e
1 changed files with 23 additions and 11 deletions

34
doc/src/sgml/func.sgml

@ -1,5 +1,5 @@
<!-- <!--
$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.295 2005/11/19 19:44:54 tgl Exp $ $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.296 2005/11/28 23:18:48 momjian Exp $
PostgreSQL documentation PostgreSQL documentation
--> -->
@ -7227,6 +7227,14 @@ SELECT ... WHERE CASE WHEN x &lt;&gt; 0 THEN y/x &gt; 1.5 ELSE false END;
<primary>COALESCE</primary> <primary>COALESCE</primary>
</indexterm> </indexterm>
<indexterm>
<primary>NVL</primary>
</indexterm>
<indexterm>
<primary>IFNULL</primary>
</indexterm>
<synopsis> <synopsis>
<function>COALESCE</function>(<replaceable>value</replaceable> <optional>, ...</optional>) <function>COALESCE</function>(<replaceable>value</replaceable> <optional>, ...</optional>)
</synopsis> </synopsis>
@ -7234,9 +7242,8 @@ SELECT ... WHERE CASE WHEN x &lt;&gt; 0 THEN y/x &gt; 1.5 ELSE false END;
<para> <para>
The <function>COALESCE</function> function returns the first of its The <function>COALESCE</function> function returns the first of its
arguments that is not null. Null is returned only if all arguments arguments that is not null. Null is returned only if all arguments
are null. This is often useful to substitute a are null. It is often used to substitute a default value for
default value for null values when data is retrieved for display, null values when data is retrieved for display, for example:
for example:
<programlisting> <programlisting>
SELECT COALESCE(description, short_description, '(none)') ... SELECT COALESCE(description, short_description, '(none)') ...
</programlisting> </programlisting>
@ -7246,7 +7253,9 @@ SELECT COALESCE(description, short_description, '(none)') ...
Like a <token>CASE</token> expression, <function>COALESCE</function> will Like a <token>CASE</token> expression, <function>COALESCE</function> will
not evaluate arguments that are not needed to determine the result; not evaluate arguments that are not needed to determine the result;
that is, arguments to the right of the first non-null argument are that is, arguments to the right of the first non-null argument are
not evaluated. not evaluated. This SQL-standard function provides capabilities similar
to <function>NVL</> and <function>IFNULL</>, which are used in some other
database systems.
</para> </para>
</sect2> </sect2>
@ -7262,16 +7271,19 @@ SELECT COALESCE(description, short_description, '(none)') ...
</synopsis> </synopsis>
<para> <para>
The <function>NULLIF</function> function returns a null value if and only The <function>NULLIF</function> function returns a null value if
if <replaceable>value1</replaceable> and <replaceable>value1</replaceable> and <replaceable>value2</replaceable>
<replaceable>value2</replaceable> are equal. Otherwise it returns are equal; otherwise it returns <replaceable>value1</replaceable>.
<replaceable>value1</replaceable>. This can be used to perform the This can be used to perform the inverse operation of the
inverse operation of the <function>COALESCE</function> example <function>COALESCE</function> example given above:
given above:
<programlisting> <programlisting>
SELECT NULLIF(value, '(none)') ... SELECT NULLIF(value, '(none)') ...
</programlisting> </programlisting>
</para> </para>
<para>
If <replaceable>value1</replaceable> is <literal>(none)</>, return a null,
otherwise return <replaceable>value1</replaceable>.
</para>
</sect2> </sect2>