PostgreSQL 7.4beta4 Documentation
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# 7.4. Combining Queries

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 10.5.

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