inner join is intersection, full outer join is union. i.e., "A inner join B an A.a = B.b" gives all combinations of rows where A's a column is not null, B's b column is not null, and they are the same. "A full outer join B on A.a = B.b" in addition gives all rows where A has an a that B doesn't have, or B has a b that A doesn't have -- the fields of the table that has no match are set to null in the resulting records in these cases.