What is a sub query and what are the different types of subqueries?

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:

Correlated sub query

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.

Non-Correlated subquery

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