exists和not exists关键字

表A
ID NAME
1    A1
2    A2
3  A3

表B
ID AID NAME
1    1 B1
2    2 B2
3    2 B3

SELECT ID,NAME FROM A WHERE EXIST (SELECT * FROM B WHERE A.ID=B.AID)

等价于 select id.name from A where A.ID IN (SELECT B.ID FROM B)
执行结果为
1 A1
2 A2

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

应用场景：

--不等于

select * from udf_user u where not exists (

        select * from armc_user_team t where t.user_name=u.username_ and t.team_code='DCCDJP'

        )

        

 --同时存在

 select * from armc_user_team t where t.team_code='a'

 and exists(select * from armc_user_team t1 where t1.user_name=t.user_name and t1.team_code='b')