A subquery is a query expression that can be used in
As an expression:
The subquery must return a single-column derived table with a maximum of one row. The value of the subquery is then the value in the derived table or the NULL value if the derived table is empty.predicates:
In the predicates ANY, SOME, ALL, IN and EXISTS the subquery returns a derived table.In the FROM clause of SELECT expressions:
The subquery returns a derived table.In join expressions:
The subquery returns a derived table.
A subquery is always enclosed in parentheses.
subquery ::= ( query_expression )
query_expression
Query expression that returns the derived table.
In subqueries that are not specified in the predicate EXISTS or in a FROM clause, the derived table can only contain an atomic column or multiple columns with the dimension 1.