What is derived table?

Derived tables are SELECT statements in the FROM clause referred to by an alias or a user-specified name. The result set of the SELECT in the FROM clause forms a table used by the outer SELECT statement.

For example, this SELECT uses a derived table to find if any store carries all book titles in the pubs database:

SELECT ST.stor_id, ST.stor_name
FROM stores AS ST,
(SELECT stor_id, COUNT(DISTINCT title_id) AS title_count
FROM sales
GROUP BY stor_id
) AS SA
WHERE ST.stor_id = SA.stor_id
AND SA.title_count = (SELECT COUNT(*) FROM titles)