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Doc: improve description of UNION/INTERSECT/EXCEPT syntax. 

queries.sgml failed to mention the rather important point that
INTERSECT binds more tightly than UNION or EXCEPT.  I thought
it could also use more discussion of the role of parentheses
in these constructs.

Per gripe from Christopher Painter-Wakefield.

Discussion: https://postgr.es/m/163338891727.12510.3939775743980651160@wrigleys.postgresql.org
This commit is contained in:
Tom Lane 2021-10-05 10:24:14 -04:00
parent 36b58ac6f6
commit b6cf89b025
1 changed files with 43 additions and 10 deletions
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53
doc/src/sgml/queries.sgml
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@ -1649,17 +1649,9 @@ SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceab
<replaceable>query1</replaceable> INTERSECT <optional>ALL</optional> <replaceable>query2</replaceable> <replaceable>query1</replaceable> INTERSECT <optional>ALL</optional> <replaceable>query2</replaceable>
<replaceable>query1</replaceable> EXCEPT <optional>ALL</optional> <replaceable>query2</replaceable> <replaceable>query1</replaceable> EXCEPT <optional>ALL</optional> <replaceable>query2</replaceable>
</synopsis> </synopsis>
<replaceable>query1</replaceable> and where <replaceable>query1</replaceable> and
<replaceable>query2</replaceable> are queries that can use any of <replaceable>query2</replaceable> are queries that can use any of
the features discussed up to this point. Set operations can also the features discussed up to this point.
be nested and chained, for example
<synopsis>
<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable> UNION <replaceable>query3</replaceable>
</synopsis>
which is executed as:
<synopsis>
(<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable>) UNION <replaceable>query3</replaceable>
</synopsis>
</para> </para>
<para> <para>
@ -1693,6 +1685,47 @@ SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceab
the corresponding columns have compatible data types, as the corresponding columns have compatible data types, as
described in <xref linkend="typeconv-union-case"/>. described in <xref linkend="typeconv-union-case"/>.
</para> </para>
<para>
Set operations can be combined, for example
<synopsis>
<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable> EXCEPT <replaceable>query3</replaceable>
</synopsis>
which is equivalent to
<synopsis>
(<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable>) EXCEPT <replaceable>query3</replaceable>
</synopsis>
As shown here, you can use parentheses to control the order of
evaluation. Without parentheses, <literal>UNION</literal>
and <literal>EXCEPT</literal> associate left-to-right,
but <literal>INTERSECT</literal> binds more tightly than those two
operators. Thus
<synopsis>
<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable> INTERSECT <replaceable>query3</replaceable>
</synopsis>
means
<synopsis>
<replaceable>query1</replaceable> UNION (<replaceable>query2</replaceable> INTERSECT <replaceable>query3</replaceable>)
</synopsis>
You can also surround an individual <replaceable>query</replaceable>
with parentheses. This is important if
the <replaceable>query</replaceable> needs to use any of the clauses
discussed in following sections, such as <literal>LIMIT</literal>.
Without parentheses, you'll get a syntax error, or else the clause will
be understood as applying to the output of the set operation rather
than one of its inputs. For example,
<synopsis>
SELECT a FROM b UNION SELECT x FROM y LIMIT 10
</synopsis>
is accepted, but it means
<synopsis>
(SELECT a FROM b UNION SELECT x FROM y) LIMIT 10
</synopsis>
not
<synopsis>
SELECT a FROM b UNION (SELECT x FROM y LIMIT 10)
</synopsis>
</para>
</sect1> </sect1>