You can use a correlated subquery to find the types of books that are published by more than one publisher:
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 rows affected)
Correlation names are required in the following query to distinguish between the two roles in which the titles table appears. This nested query is equivalent to the self-join query:
select distinct t1.type from titles t1, titles t2 where t1.type = t2.type and t1.pub_id != t2.pub_id
