Since a subquery is just an expression that appears in the WHERE or HAVING clauses, the search conditions on subqueries may look familiar.
Comparison predicates using subqueries Compares the value of an expression to a single value produced by the subquery for each record in the table(s) in the main query. Comparison tests use the operators (=, <>, <. <=, >, >=) provided with the subquery.
Quantified comparison test Compares the value of an expression to each of the set of values produced by a subquery.
Subquery set membership test Checks if the value of an expression matches one of the set of values produced by a subquery.
Existence test Checks if the subquery produces any rows.
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