select d.dname "부서명", e2.empno "관리자번호", e1.c "사원수" from dept d LEFT OUTER JOIN (select count(*) c, deptno from emp group by deptno) e1 on d.deptno = e1.deptno LEFT OUTER JOIN (select deptno, empno from emp where mgr in (select empno from emp where mgr is null)) e2 on d.deptno = e2.deptno group by d.dname, e2.empno, e1.c; outer join을 이용하는 것을 알았지만, 왜 1번 하는 것까지만 생각했을까? 조인에 대해서 잘 이해를 하고 있어야 더 나아가는 게..