부서별로 급여가 2000 이상인 사원 수를 세려면?

1. 1SELECT deptno, COUNT(sal >= 2000) FROM emp GROUP BY deptno
2. 2SELECT deptno, COUNT(*) FROM emp WHERE sal >= 2000 GROUP BY deptno
3. 3SELECT deptno, SUM(CASE WHEN sal >= 2000 THEN 1 ELSE 0 END) FROM emp GROUP BY deptno
4. 4②와 ③이 모두 유효하다