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For example, to find the names of publishers who do not publish business books, the query is: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 (1 row affected)
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 (2 rows affected)
