Subquery comparison test   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.