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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 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 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.


 
 
  Published courtesy of The PostgreSQL Global Development Group Design by Interspire