Co-related sub-query:- The main differencre between subquery and co-related subquery is that in subquery child query is executed first n then parent but in co-related subquery main query is executed first(even though parenthesis are present) and then child query.

Example of co-related subquery

select dname from dept where exists
(select deptno from emp
where dept.deptno emp.deptno);

select dname from dept where not exists
(select deptno from emp
where dept.deptno emp.deptno);