The results of two queries can be combined using the set operations union, intersection, and difference. The syntax is
query2 are queries that
can use any of the features discussed up to this point. Set
operations can also be nested and chained, for example
which is executed as:
UNION effectively appends the
query2 to the
(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
ALL is used.
INTERSECT returns all rows that
are both in the result of
query1 and in the result of
query2. Duplicate rows
are eliminated unless
EXCEPT returns all rows that are
in the result of
but not in the result of
query2. (This is sometimes called
the difference between two queries.)
Again, duplicates are eliminated unless
EXCEPT ALL is used.
In order to calculate the union, intersection, or difference of two queries, the two queries must be “union compatible”, which means that they return the same number of columns and the corresponding columns have compatible data types, as described in Section 10.5.
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