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体验新版 GitCode,发现更多精彩内容 >>
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ee4aa302
编写于
11月 28, 2005
作者:
B
Bruce Momjian
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差异文件
Improve documentation for COALESCE and NULLIF. Add references for NVL
and IFNULL. Backpatch to 8.1.X.
上级
20054503
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1
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23 addition
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doc/src/sgml/func.sgml
doc/src/sgml/func.sgml
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doc/src/sgml/func.sgml
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ee4aa302
<!--
<!--
$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.29
5 2005/11/19 19:44:54 tgl
Exp $
$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.29
6 2005/11/28 23:18:48 momjian
Exp $
PostgreSQL documentation
PostgreSQL documentation
-->
-->
...
@@ -7227,6 +7227,14 @@ SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;
...
@@ -7227,6 +7227,14 @@ SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 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 <> 0 THEN y/x > 1.5 ELSE false END;
...
@@ -7234,9 +7242,8 @@ SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 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)') ...
...
@@ -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)') ...
...
@@ -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>
...
...
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