A subquery is an SQL statement specifying a search condition that contains either a subselect or a fullselect, and is used inside another SQL statement. A subquery can include search conditions of its own. Each search condition can also contain subqueries. Any SQL statement can contain a hierarchy of subqueries. The SQL query block that contains subqueries is at a higher level than the subqueries they contain. This is important for how columns are referenced between levels.

A subquery can contain search conditions that reference not only columns of the tables identified by its FROM clause, but also columns of tables identified at any higher level. The reverse, however, is not true. A query at a higher level cannot reference columns ...

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