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.
For example, to find the names of publishers who do not publish business books, the query is:
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
