not exists is just like exists except that the where clause in which it is used is satisfied when no rows are returned by the subquery.
select pub_name from publishers where not exists (select * from titles where pub_id = publishers.pub_id and type = "business")
pub_name ---------------------------------------- Binnet & Hardley
This query finds the titles for which there have been no sales:
select title from titles where not exists (select title_id from salesdetail where title_id = titles.title_id)
title ----------------------------------------- The Psychology of Computer Cooking Net Etiquette