diff options
| -rw-r--r-- | doc/src/sgml/func.sgml | 27 | ||||
| -rw-r--r-- | doc/src/sgml/typeconv.sgml | 12 |
2 files changed, 33 insertions, 6 deletions
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index 7b1dc264f68..2783985b552 100644 --- a/doc/src/sgml/func.sgml +++ b/doc/src/sgml/func.sgml @@ -17287,6 +17287,12 @@ 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 @@ -17316,13 +17322,30 @@ 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"> diff --git a/doc/src/sgml/typeconv.sgml b/doc/src/sgml/typeconv.sgml index cfeb851a507..810b0cb5573 100644 --- a/doc/src/sgml/typeconv.sgml +++ b/doc/src/sgml/typeconv.sgml @@ -127,8 +127,10 @@ must appear in a single set of columns, the types of the results of each <command>SELECT</command> clause must be matched up and converted to a uniform set. Similarly, the result expressions of a <literal>CASE</literal> construct must be converted to a common type so that the <literal>CASE</literal> expression as a whole -has a known output type. The same holds for <literal>ARRAY</literal> constructs, -and for the <function>GREATEST</function> and <function>LEAST</function> functions. +has a known output type. Some other constructs, such +as <literal>ARRAY[]</literal> and the <function>GREATEST</function> +and <function>LEAST</function> functions, likewise require determination of a +common type for several subexpressions. </para> </listitem> </varlistentry> @@ -1040,9 +1042,11 @@ SQL <literal>UNION</literal> constructs must match up possibly dissimilar types to become a single result set. The resolution algorithm is applied separately to each output column of a union query. The <literal>INTERSECT</literal> and <literal>EXCEPT</literal> constructs resolve -dissimilar types in the same way as <literal>UNION</literal>. The +dissimilar types in the same way as <literal>UNION</literal>. +Some other constructs, including <literal>CASE</literal>, <literal>ARRAY</literal>, <literal>VALUES</literal>, -<function>GREATEST</function> and <function>LEAST</function> constructs use the identical +and the <function>GREATEST</function> and <function>LEAST</function> +functions, use the identical algorithm to match up their component expressions and select a result data type. </para> |