The results of two queries can be combined using the set operations union, intersection, and difference. The syntax is
comando1 UNION [ALL] comando2 comando1 INTERSECT [ALL] comando2 comando1 EXCEPT [ALL] comando2
comando1 and comando2 are queries that can use any of the features discussed up to this point. Set operations can also be nested and chained, for example
comando1 UNION comando2 UNION comando3
which really says
(comando1 UNION comando2) UNION comando3
UNION effectively appends the result of comando2 to the result of comando1 (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 comando1 and in the result of comando2. Duplicate rows are eliminated unless INTERSECT ALL is used.
EXCEPT returns all rows that are in the result of comando1 but not in the result of comando2. (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 Seção 10.5.
