Editing Select (SQL) (section)

=== Subqueries ===

Queries can be nested so that the results of one query can be used in another query via a [[relational operator]] or aggregation function. A nested query is also known as a ''subquery''. While joins and other table operations provide computationally superior (i.e. faster) alternatives in many cases (all depending on implementation), the use of subqueries introduces a hierarchy in execution that can be useful or necessary. In the following example, the aggregation function <code>AVG</code> receives as input the result of a subquery:

<syntaxhighlight lang="sql">
SELECT isbn,
       title,
       price
 FROM  Book
 WHERE price < (SELECT AVG(price) FROM Book)
 ORDER BY title;
</syntaxhighlight>

A subquery can use values from the outer query, in which case it is known as a [[correlated subquery]].

Since 1999 the SQL standard allows WITH clauses, i.e. named subqueries often called [[common table expression]]s (named and designed after the IBM DB2 version 2 implementation; Oracle calls these [[subquery factoring]]). CTEs can also be [[recursive]] by referring to themselves; [[Hierarchical and recursive queries in SQL|the resulting mechanism]] allows tree or graph traversals (when represented as relations), and more generally [[fixpoint]] computations.