</para>

</example>

<example>

<title>Type Resolution in a Nested Union</title>

<para>

<screen>

SELECT NULL UNION SELECT NULL UNION SELECT 1;

ERROR: UNION types text and integer cannot be matched

</screen>

This failure occurs because <productname>PostgreSQL</productname> treats

multiple <literal>UNION</literal>s as a nest of pairwise operations;

that is, this input is the same as

<screen>

(SELECT NULL UNION SELECT NULL) UNION SELECT 1;

</screen>

The inner <literal>UNION</literal> is resolved as emitting

type <type>text</type>, according to the rules given above. Then the

outer <literal>UNION</literal> has inputs of types <type>text</type>

and <type>integer</type>, leading to the observed error. The problem

can be fixed by ensuring that the leftmost <literal>UNION</literal>

has at least one input of the desired result type.

</para>

<para>

<literal>INTERSECT</literal> and <literal>EXCEPT</literal> operations are

likewise resolved pairwise. However, the other constructs described in this

section consider all of their inputs in one resolution step.

</para>

</example>

</sect1>

</chapter>