<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>

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>