exists和not exists用法
2011-05-11 10:58:44| 分类:
数据库、表、视图
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exists (sql 返回结果集,为真)
not exists (sql 不返回结果集,为真)
如下:
表A
ID NAME
1 A1
2 A2
3 A3
表B
ID AID NAME
1 1 B1
2 2 B2
3 2 B3
表A和表B是1对多的关系 A.
ID => B.AID
SELECT ID,NAME FROM A WHERE EXIST (SELECT * FROM B WHERE A.
ID=B.AID)
执行结果为
1 A1
2 A2
原因可以按照如下分析
SELECT ID,NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.
AID=1)
---> SELECT * FROM B WHERE B.
AID=1有值,返回真,所以有数据
SELECT ID,NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.
AID=2)
---> SELECT * FROM B WHERE B.
AID=2有值,返回真,所以有数据
SELECT ID,NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.
AID=3)
---> SELECT * FROM B WHERE B.
AID=3无值,返回假,所以没有数据
NOT EXISTS 就是反过来
SELECT ID,NAME FROM A WHERE NOT EXIST (SELECT * FROM B WHERE A.
ID=B.AID)
执行结果为
3 A3
EXISTS = IN,意思相同不过语法上有点点区别,好像使用IN效率要差点,应该是不会执行索引的原因
SELECT ID,NAME FROM A WHERE ID IN (SELECT AID FROM B)
NOT EXISTS = NOT IN ,意思相同不过语法上有点点区别
SELECT ID,NAME FROM A WHERE ID NOT IN (SELECT AID FROM B)
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