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<title>7.4.Combining Queries</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="queries-union"></a>7.4.Combining Queries</h2></div></div></div>
<a name="id582820"></a><a name="id582831"></a><a name="id582841"></a><a name="id582852"></a><a name="id582862"></a><a name="id582872"></a><a name="id582882"></a><p> The results of two queries can be combined using the set operations
union, intersection, and difference. The syntax is
</p>
<pre class="synopsis"><em class="replaceable"><code>query1</code></em> UNION [<span class="optional">ALL</span>] <em class="replaceable"><code>query2</code></em>
<em class="replaceable"><code>query1</code></em> INTERSECT [<span class="optional">ALL</span>] <em class="replaceable"><code>query2</code></em>
<em class="replaceable"><code>query1</code></em> EXCEPT [<span class="optional">ALL</span>] <em class="replaceable"><code>query2</code></em></pre>
<p>
<em class="replaceable"><code>query1</code></em> and
<em class="replaceable"><code>query2</code></em> are queries that can use any of
the features discussed up to this point. Set operations can also
be nested and chained, for example
</p>
<pre class="synopsis"><em class="replaceable"><code>query1</code></em> UNION <em class="replaceable"><code>query2</code></em> UNION <em class="replaceable"><code>query3</code></em></pre>
<p>
which really says
</p>
<pre class="synopsis">(<em class="replaceable"><code>query1</code></em> UNION <em class="replaceable"><code>query2</code></em>) UNION <em class="replaceable"><code>query3</code></em></pre>
<p>
</p>
<p> <code class="literal">UNION</code> effectively appends the result of
<em class="replaceable"><code>query2</code></em> to the result of
<em class="replaceable"><code>query1</code></em> (although there is no guarantee
that this is the order in which the rows are actually returned).
Furthermore, it eliminates duplicate rows from its result, in the same
way as <code class="literal">DISTINCT</code>, unless <code class="literal">UNION ALL</code> is used.
</p>
<p> <code class="literal">INTERSECT</code> returns all rows that are both in the result
of <em class="replaceable"><code>query1</code></em> and in the result of
<em class="replaceable"><code>query2</code></em>. Duplicate rows are eliminated
unless <code class="literal">INTERSECT ALL</code> is used.
</p>
<p> <code class="literal">EXCEPT</code> returns all rows that are in the result of
<em class="replaceable"><code>query1</code></em> but not in the result of
<em class="replaceable"><code>query2</code></em>. (This is sometimes called the
<em class="firstterm">difference</em> between two queries.) Again, duplicates
are eliminated unless <code class="literal">EXCEPT ALL</code> is used.
</p>
<p> In order to calculate the union, intersection, or difference of two
queries, the two queries must be “<span class="quote">union compatible</span>”,
which means that they return the same number of columns and
the corresponding columns have compatible data types, as
described in <a href="typeconv-union-case.html" title="10.5.UNION, CASE, and Related Constructs">Section10.5, “<code class="literal">UNION</code>, <code class="literal">CASE</code>, and Related Constructs”</a>.
</p>
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