Subquery :- The inner query is executed only once The inner query will get executed first and the output of the inner query used by the outer query.The inner query is not dependent on outer query.
Eg:- SELECT cust_name, dept_no FROM Customer WHERE cust_name IN (SELECT cust_name FROM Customer);
Correlated subquery:-The outer query will get executed first and for every row of outer query, inner query will get executed. So the inner query will get executed as many times as no.of rows in result of the outer query.The outer query output can use the inner query output for comparison. This means inner query and outer query dependent on each other
Eg: SELECT cust_name,dept_id FROM Cust WHERE cust_name in (SELECT cust_name FROM dept WHERE cust.dept_id=dept.dept_id);