lmgtfy, for

PostgreSQL, Oracle, and SQLite.

As chacham said, UNION ALL leaves the rowsets from all query-parts intact; UNION is slightly more expensive, as it makes the final rows DISTINCT (i.e., removes duplicates).

With regard to these compound query operators, note that Oracle stubbornly uses MINUS instead of the Standard-prescribed EXCEPT. Ah well, I guess it's too late to teach them manners ;-)