A subquery places one query inside another one. The second query resides somewhere within the WHERE clause of a SELECT statement. One or more values returned by the subquery are used by the main query to return the results to the user
Subquery: A query that is embedded in a main, or parent, query and used to assist in filtering the result set from a query.
The types of operators allowed in the WHERE clause depend on whether the subquery returns one row or more than one row. If only a single row is returned from a query, the comparison operators =, !=, <, >, ?, ?, and so forth are valid. If more than one row is returned from a subquery, operators such as IN, NOT IN, ANY, and ALL are valid.
