SQL优化常用方法18

1. 用EXISTS替代IN
   在许多基于基础表的查询中,为了满足一个条件,往往需要对另一个表进行联接.在这种情况下, 使用EXISTS(或NOT EXISTS)通常将提高查询的效率. 低效:

SELECT * FROM EMP (基础表)
WHERE EMPNO > 0 AND DEPTNO IN (SELECT DEPTNO FROM DEPT WHERE LOC = ‘MELB')
高效: SELECT * FROM EMP (基础表)
WHERE EMPNO > 0 AND EXISTS (SELECT ‘X' FROM DEPT WHERE DEPT.DEPTNO = EMP.DEPTNO AND LOC = ‘MELB')