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无值返回真所以没有数据
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