SQL中EXISTS怎么用

Question

如题所述

Answer 1

EXISTS 是判断是否存在,和in类似,但效率要比in高
SELECT * FROM EMP (基础表) WHERE EMPNO > 0 AND EXISTS (SELECT 'X' FROM DEPT WHERE DEPT.DEPTNO = EMP.DEPTNO AND LOC = 'MELB')

SELECT * FROM EMP (基础表) WHERE EMPNO > 0 AND DEPTNO IN(SELECT DEPTNO FROM DEPT WHERE LOC = 'MELB')