-- FROM 절에 서브쿼리
select * from emp where deptno=30;
select * from (select * from emp where deptno=30) where deptno=20;
select * from (select * from emp where sal > (select avg(sal) from emp where deptno=20)) a, emp b
where a.empno=b.empno and b.mgr is not null and b.deptno!=20;
