Information Modeling and Relational Databases, 2nd Edition

12.11. SQL: Correlated and Existential Subqueries

This section extends the subquery work in Section 12.8 by considering correlated and existential subqueries. The subqueries discussed earlier were simple, uncorrelated sub-queries. An uncorrelated subquery is computed once, replaced by its result, and the outer query is run. In contrast, a correlated subquery relates its search condition to each row of a table named in the outer query and is effectively recomputed for each outer row (see Figure 12.58).

Figure 12.58. A correlated subquery relates its condition to each row of the outer query.

Because of repeated computation, correlated subqueries ...