7.4. Combining Queries
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.
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
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.
Set operations can be combined, for example
which is equivalent to
As shown here, you can use parentheses to control the order of evaluation. Without parentheses,
EXCEPT associate left-to-right, but
INTERSECT binds more tightly than those two operators. Thus
You can also surround an individual
query with parentheses. This is important if the
query needs to use any of the clauses discussed in following sections, such as
LIMIT. Without parentheses, you'll get a syntax error, or else the clause will be understood as applying to the output of the set operation rather than one of its inputs. For example,
SELECT a FROM b UNION SELECT x FROM y LIMIT 10
is accepted, but it means
(SELECT a FROM b UNION SELECT x FROM y) LIMIT 10
SELECT a FROM b UNION (SELECT x FROM y LIMIT 10)