oracle数据库下关键字EXIST，NOT EXISTS使用讲解

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