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 all duplicate rows, in the sense of
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 both return the same number of columns, and
that the corresponding columns have compatible data types, as
described in Section 7.5.
