A subquery is a query(expressed as a SELECT statement) that is located inside another query. The first SQL statement is known as the outer query, the second is known as the inner query or subquery. The inner query or subquery is normally executed first. the output of the inner query is used as the input for the outer query. A subquery is normally expressed inside parenthesis and can return zero, one or more rows and each row can have one or more columns.