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doc: move ROW IS NULL examples to a different chapter

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Also add examples.

Reported-by: Wolfgang Walther

Discussion: https://postgr.es/m/21ff8e9c-627a-f949-fb00-a41b9ddcc9d3@technowledgy.de

Backpatch-through: master
This commit is contained in:
Bruce Momjian 2023-11-13 15:20:54 -05:00
parent 8680bae846
commit bd86407892
2 changed files with 19 additions and 12 deletions
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14
doc/src/sgml/func.sgml
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@ -718,7 +718,19 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
<literal>IS NULL</literal> and <literal>IS NOT NULL</literal> do not always return
inverse results for row-valued expressions; in particular, a row-valued
expression that contains both null and non-null fields will return false
for both tests. In some cases, it may be preferable to
for both tests. For example:
<programlisting>
SELECT ROW(1,2.5,'this is a test') = ROW(1, 3, 'not the same');
SELECT ROW(table.*) IS NULL FROM table; -- detect all-null rows
SELECT ROW(table.*) IS NOT NULL FROM table; -- detect all-non-null rows
SELECT NOT(ROW(table.*) IS NOT NULL) FROM TABLE; -- detect at least one null in rows
</programlisting>
In some cases, it may be preferable to
write <replaceable>row</replaceable> <literal>IS DISTINCT FROM NULL</literal>
or <replaceable>row</replaceable> <literal>IS NOT DISTINCT FROM NULL</literal>,
which will simply check whether the overall row value is null without any

17
doc/src/sgml/syntax.sgml
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@ -2479,17 +2479,12 @@ SELECT getf1(CAST(ROW(11,'this is a test',2.5) AS myrowtype));
<para>
Row constructors can be used to build composite values to be stored
in a composite-type table column, or to be passed to a function that
accepts a composite parameter. Also,
it is possible to compare two row values or test a row with
<literal>IS NULL</literal> or <literal>IS NOT NULL</literal>, for example:
<programlisting>
SELECT ROW(1,2.5,'this is a test') = ROW(1, 3, 'not the same');
SELECT ROW(table.*) IS NULL FROM table; -- detect all-null rows
</programlisting>
For more detail see <xref linkend="functions-comparisons"/>.
Row constructors can also be used in connection with subqueries,
as discussed in <xref linkend="functions-subquery"/>.
accepts a composite parameter. Also, it is possible to test rows
using the standard comparison operators as described in <xref
linkend="functions-comparison"/>, to compare one row against another
as described in <xref linkend="functions-comparisons"/>, and to
use them in connection with subqueries, as discussed in <xref
linkend="functions-subquery"/>,
</para>
</sect2>