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

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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 f6b5d05ba9
commit f3fec23dbd
1 changed files with 43 additions and 10 deletions
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53
doc/src/sgml/queries.sgml
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@ -1707,17 +1707,9 @@ SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceab
<replaceable>query1</replaceable> INTERSECT <optional>ALL</optional> <replaceable>query2</replaceable>
<replaceable>query1</replaceable> EXCEPT <optional>ALL</optional> <replaceable>query2</replaceable>
</synopsis>
<replaceable>query1</replaceable> and
where <replaceable>query1</replaceable> and
<replaceable>query2</replaceable> are queries that can use any of
the features discussed up to this point. Set operations can also
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>
the features discussed up to this point.
</para>
<para>
@ -1751,6 +1743,47 @@ SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceab
the corresponding columns have compatible data types, as
described in <xref linkend="typeconv-union-case"/>.
</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>