diff options
Diffstat (limited to 'doc/src/sgml/func.sgml')
| -rw-r--r-- | doc/src/sgml/func.sgml | 27 |
1 files changed, 25 insertions, 2 deletions
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index f055973491b..c123b7fc828 100644 --- a/doc/src/sgml/func.sgml +++ b/doc/src/sgml/func.sgml @@ -12639,6 +12639,12 @@ SELECT COALESCE(description, short_description, '(none)') ... </para> <para> + The arguments must all be convertible to a common data type, which + will be the type of the result (see + <xref linkend="typeconv-union-case"/> for details). + </para> + + <para> Like a <token>CASE</token> expression, <function>COALESCE</function> only evaluates the arguments that are needed to determine the result; that is, arguments to the right of the first non-null argument are @@ -12668,13 +12674,30 @@ SELECT COALESCE(description, short_description, '(none)') ... <programlisting> SELECT NULLIF(value, '(none)') ... </programlisting> - </para> - <para> In this example, if <literal>value</literal> is <literal>(none)</literal>, null is returned, otherwise the value of <literal>value</literal> is returned. </para> + <para> + The two arguments must be of comparable types. + To be specific, they are compared exactly as if you had + written <literal><replaceable>value1</replaceable> + = <replaceable>value2</replaceable></literal>, so there must be a + suitable <literal>=</literal> operator available. + </para> + + <para> + The result has the same type as the first argument — but there is + a subtlety. What is actually returned is the first argument of the + implied <literal>=</literal> operator, and in some cases that will have + been promoted to match the second argument's type. For + example, <literal>NULLIF(1, 2.2)</literal> yields <type>numeric</type>, + because there is no <type>integer</type> <literal>=</literal> + <type>numeric</type> operator, + only <type>numeric</type> <literal>=</literal> <type>numeric</type>. + </para> + </sect2> <sect2 id="functions-greatest-least"> |