Subqueries that are introduced with the keyword phrase not in also return a list of values that are zero (0) and greater.
not in means “not = a and not = b and not = c.”
This query finds the names of the publishers who have not published business books:
select pub_name from publishers where pub_id not in (select pub_id from titles where type = "business")
pub_name ---------------------------------------- Binnet & Hardley
The query is the same as the previous one except that not in is substituted for in. However, you cannot convert this statement to a join; the “not equal” join finds the names of publishers who have published some book that is not a business book.