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><A
NAME="QUERIES-UNION"
>7.4. Combining Queries</A
></H1
><P
> The results of two queries can be combined using the set operations
union, intersection, and difference. The syntax is
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>query1</I
></TT
> UNION [<SPAN
CLASS="OPTIONAL"
>ALL</SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>query2</I
></TT
>
<TT
CLASS="REPLACEABLE"
><I
>query1</I
></TT
> INTERSECT [<SPAN
CLASS="OPTIONAL"
>ALL</SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>query2</I
></TT
>
<TT
CLASS="REPLACEABLE"
><I
>query1</I
></TT
> EXCEPT [<SPAN
CLASS="OPTIONAL"
>ALL</SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>query2</I
></TT
></PRE
><P>
<TT
CLASS="REPLACEABLE"
><I
>query1</I
></TT
> and
<TT
CLASS="REPLACEABLE"
><I
>query2</I
></TT
> 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"
><TT
CLASS="REPLACEABLE"
><I
>query1</I
></TT
> UNION <TT
CLASS="REPLACEABLE"
><I
>query2</I
></TT
> UNION <TT
CLASS="REPLACEABLE"
><I
>query3</I
></TT
></PRE
><P>
which is executed as:
</P><PRE
CLASS="SYNOPSIS"
>(<TT
CLASS="REPLACEABLE"
><I
>query1</I
></TT
> UNION <TT
CLASS="REPLACEABLE"
><I
>query2</I
></TT
>) UNION <TT
CLASS="REPLACEABLE"
><I
>query3</I
></TT
></PRE
><P>
</P
><P
> <TT
CLASS="LITERAL"
>UNION</TT
> effectively appends the result of
<TT
CLASS="REPLACEABLE"
><I
>query2</I
></TT
> to the result of
<TT
CLASS="REPLACEABLE"
><I
>query1</I
></TT
> (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 <TT
CLASS="LITERAL"
>DISTINCT</TT
>, unless <TT
CLASS="LITERAL"
>UNION ALL</TT
> is used.
</P
><P
> <TT
CLASS="LITERAL"
>INTERSECT</TT
> returns all rows that are both in the result
of <TT
CLASS="REPLACEABLE"
><I
>query1</I
></TT
> and in the result of
<TT
CLASS="REPLACEABLE"
><I
>query2</I
></TT
>. Duplicate rows are eliminated
unless <TT
CLASS="LITERAL"
>INTERSECT ALL</TT
> is used.
</P
><P
> <TT
CLASS="LITERAL"
>EXCEPT</TT
> returns all rows that are in the result of
<TT
CLASS="REPLACEABLE"
><I
>query1</I
></TT
> but not in the result of
<TT
CLASS="REPLACEABLE"
><I
>query2</I
></TT
>. (This is sometimes called the
<I
CLASS="FIRSTTERM"
>difference</I
> between two queries.) Again, duplicates
are eliminated unless <TT
CLASS="LITERAL"
>EXCEPT ALL</TT
> 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"
>Section 10.5</A
>.
</P
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