Subquery restrictions

Subqueries must appear on the right side of an expression.

Nested subqueries are not supported.

A single query can have only one subquery expression.

Subquery predicates must appear as top-level conjuncts.

Subqueries support four logical operators in query predicates: IN, NOT IN, EXISTS, and NOT EXISTS.

The IN and NOT IN logical operators may select only one column in a WHERE clause subquery.

The EXISTS and NOT EXISTS operators must have at least one correlated predicate.

The left side of a subquery must qualify all references to table columns.

References to columns in the parent query are allowed only in the WHERE clause of the subquery.

Subquery predicates that reference a column in a parent query must use the equals (=) predicate operator.

Subquery predicates may not refer only to columns in the parent query.

Correlated subqueries with an implied GROUP BY statement may return only one row.

All unqualified references to columns in a subquery must resolve to tables in the subquery.

Correlated subqueries cannot contain windowing clauses.