What is the difference between sub-query & co-related sub query ?

A sub query is executed once for the parent statement whereas the correlated sub query is executed once for each row of the parent query.

Sub Query:

Example:

 Select deptno, ename, sal from emp a  where sal  in (select sal from Grade  where sal_grade=’A’ or  sal_grade=’B’)

Co-Related Sun query:

Example:

Find all employees who earn more than the average salary in their department.

SELECT last-named, salary, department_id  FROM employees A

WHERE salary > (SELECT AVG (salary)

FROM employees B WHERE B.department_id =A.department_id

Group by B.department_id)

EXISTS:

The EXISTS operator tests for existence of rows in

the results set of the subquery.

Select dname from dept where exists (select 1 from EMP where dept.deptno= emp.deptno);

Sub-query	Co-related sub-query
A sub-query is executed once for the parent Query	Where as co-related sub-query is executed once for each row of the parent query.
Example: Select * from emp where deptno in (select deptno from dept);	Example: Select a.* from emp e where sal >= (select avg(sal) from emp a where a.deptno=e.deptno group by  a.deptno);