The branch of mathematics or logic that is concerned with sets of objects and rules for their manipulation. UNION, INTERSECT and COMPLEMENT are its three primary operations and they are used in relational databases as follows.
Given a file of Americans and a file of golfers, UNION would create a file of all Americans and golfers. INTERSECT would create a file of American golfers, and COMPLEMENT would create a file of golfers who are not Americans, or of Americans who are not golfers. See fuzzy logic