You can use a correlated subquery to find the types of books that are published by more than one publisher. Aliases are required to distinguish the two different roles in which the titles table appears:
SELECT DISTINCT t1.type FROM titles t1 WHERE t1.type in (SELECT t2.type FROM titles t2 WHERE t1.pub_id <> t2.pub_id) type ---------- business psychology (2 row(s) affected)
The preceding nested query is equivalent to this self-join:
SELECT DISTINCT t1.type FROM titles t1, titles t2 WHERE t1.type = t2.type AND t1.pub_id <> t2.pub_id