|
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.
|
