Chapter 10. Correlated Subqueries

A correlated subquery is an inner subquery whose information is referenced by the main, outer query such that the inner query may be thought of as being executed repeatedly. In this chapter, we discuss correlated subqueries in detail. We discuss existence queries (EXISTS) and correlation as well as NOT EXISTS. We also take a look at SQL's universal and existential qualifiers. Before discussing correlated subqueries in detail however, let's make sure that you understand what constitutes a noncorrelated subquery.