TSQL之JOIN（连接）彻底理解（开发人员必看）

T-SQL之JOIN彻底理解

本文不解释，欢迎提问解答，谢谢。直接上代码：

--测试数据建立
Create TABLE JOIN_TEST(
COLA INT,
COLB INT
)

GO
INSERT INTO JOIN_TEST
SELECT 1,1

INSERT INTO JOIN_TEST
SELECT 1,1

GO
--测试数据建立完毕

SELECT * FROM JOIN_TEST
/*

COLA        COLB
----------- -----------
1           1
1           1

(2 行受影响)
*/


SELECT * FROM JOIN_TEST A  CROSS JOIN JOIN_TEST B
/*
COLA        COLB        COLA        COLB
----------- ----------- ----------- -----------
1           1           1           1
1           1           1           1
1           1           1           1
1           1           1           1

(4 行受影响)

*/


SELECT * FROM JOIN_TEST A  INNER JOIN  JOIN_TEST B ON A.COLA=B.COLA
/*
COLA        COLB        COLA        COLB
----------- ----------- ----------- -----------
1           1           1           1
1           1           1           1
1           1           1           1
1           1           1           1
(4 行受影响)

*/


SELECT * FROM JOIN_TEST A  LEFT JOIN  JOIN_TEST B ON A.COLA=B.COLA
/*
COLA        COLB        COLA        COLB
----------- ----------- ----------- -----------
1           1           1           1
1           1           1           1
1           1           1           1
1           1           1           1
(4 行受影响)

*/

SELECT * FROM JOIN_TEST A  LEFT JOIN  JOIN_TEST B ON A.COLA=B.COLA AND A.COLA<>1
/*
COLA        COLB        COLA        COLB
----------- ----------- ----------- -----------
1           1           NULL        NULL
1           1           NULL        NULL
(2 行受影响)
*/

SELECT * FROM JOIN_TEST A  LEFT JOIN  JOIN_TEST B ON A.COLA=B.COLA WHERE A.COLA<>1
/*
COLA        COLB        COLA        COLB
----------- ----------- ----------- -----------

(0 行受影响)
*/

SELECT * FROM JOIN_TEST A  FULL JOIN  JOIN_TEST B ON A.COLA=B.COLA AND A.COLA<>1
/*
COLA        COLB        COLA        COLB
----------- ----------- ----------- -----------
1           1           NULL        NULL
1           1           NULL        NULL
NULL        NULL        1           1
NULL        NULL        1           1
*/

SELECT * FROM JOIN_TEST A  FULL JOIN  JOIN_TEST B ON A.COLA=B.COLA where A.COLA<>1
/*
COLA        COLB        COLA        COLB
----------- ----------- ----------- -----------

(0 行受影响)
*/