EXISTS怎么替代in

1、EXISTS的使用方法

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‘)

这两句效果一样

EXISTS怎么替代in

原文：http://my.oschina.net/weiweiblog/blog/482013