Postgres offers set operators that make it easy to query and filter the results of searches from your database. Set operators are used to join the results of two or more SELECT statements. These operators are UNION, UNION ALL, INTERSECT, and EXCEPT-each can be used to construct queries across multiple tables and filter for the specific data that you need.

To return the combined results of two SELECT statements, we use the UNION set operator. This operator removes all the duplicates from the queried results-only listing one row for each duplicated result. To examine this behavior, the UNION ALL set operator can be used, as it retains duplicates in the final result. The INTERSECT set operator only lists records that are shared by both SELECT queries and conversely, the EXCEPT set operator removes the results from the second SELECT query. Thus, the INTERSECT and EXCEPT set operators are used to producing unduplicated results.

Source: DZone