CREATE TABLE testA(keyId INT)

CREATE TABLE testB(keyId INT)

GO

INSERT INTO testA(keyId) VALUES (1)

INSERT INTO testA(keyId) VALUES (2)

INSERT INTO testA(keyId) VALUES (3)

GO

INSERT INTO testB(keyId) VALUES (1)

INSERT INTO testB(keyId) VALUES (2)

INSERT INTO testB(keyId) VALUES (4)

INSERT INTO testB(keyId) VALUES (NULL)

GO

 SELECT * FROM testA WHERE keyId IN (SELECT keyId FROM testB)

-----输出------

/*

keyId

1

2

*/

 SELECT * FROM testA WHERE keyId not IN (SELECT keyId FROM testB)

------无输出记录-----

/*

keyId

*/

INSERT INTO testA VALUES (NULL) ---在testA表中插入空值

SELECT * FROM testA WHERE EXISTS(

    SELECT null  FROM testB WHERE testA.keyId=testB.keyId

)

----输出----

/*

keyId

1

2

*/

SELECT * FROM testA WHERE not  EXISTS(

    SELECT null  FROM testB WHERE testA.keyId=testB.keyId

)

/*

keyId

3

NULL

*/

SELECT * FROM testA WHERE testA.keyId NOT in (

    SELECT testB.keyId FROM testB WHERE testB.keyId IS NOT NULL

)

drop table testA

drop table testB