The results of two queries can be combined using the set operations union, intersection, and difference. The syntax is
query1
UNION [ALL]
query2
query1
INTERSECT [ALL]
query2
query1
EXCEPT [ALL]
query2
query1
and
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
query1
UNION
query2
UNION
query3
which really says
(
query1
UNION
query2
) UNION
query3
UNION effectively appends the result of
query2
to the result of
query1
(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 DISTINCT, unless UNION 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 INTERSECT ALL is used.
EXCEPT returns all rows that are in the result of
query1
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.
