Differences between sub-query and co-related sub-query

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);