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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 named subqueries 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.
=== Derived table ===
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