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=== 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, 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">
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