Sub Query is also called as Nested Query or Inner Query which is used to get data from multiple tables. A sub query is added in the where clause of the main query.
There are two different types of subqueries:
A Correlated sub query cannot be as independent query but can reference column in a table listed in the from list of the outer query.
This can be evaluated as if it were an independent query. Results of the sub query are submitted to the main query or parent query.