SQL SERVER LEFT JOIN, INNER JOIN, RIGHT JOIN

• JOIN: 如果表中有至少一个匹配，则返回行
• LEFT JOIN: 即使右表中没有匹配，也从左表返回所有的行
• RIGHT JOIN: 即使左表中没有匹配，也从右表返回所有的行
• FULL JOIN: 只要其中一个表中存在匹配，就返回行

CREATE TABLE dbo.Student
(
 Sno int  null
,Name nvarchar(23)
)
CREATE TABLE dbo.Score
(
 Sno int
,Score INT
)
INSERT INTO dbo.Student(Sno,Name)VALUES(1,'Jesse');
INSERT INTO dbo.Student(Sno,Name)VALUES(2,'Jessca');
INSERT INTO dbo.Student(Sno,Name)VALUES(3,'June');
INSERT INTO dbo.Student(Sno,Name)VALUES(4,'Supper');

INSERT INTO dbo.Score(Sno,Score)VALUES (1,45)
INSERT INTO dbo.Score(Sno,Score)VALUES (1,56)
INSERT INTO dbo.Score(Sno,Score)VALUES (2,100)
INSERT INTO dbo.Score(Sno,Score)VALUES (3,25)  

　查看数据：

SELECT * FROM dbo.Student
SELECT * FROM dbo.Score

　　

Left Join 会显示出左边的全部数据，右边如果没有对应的值，系统默认NULL

SELECT *
FROM dbo.Student a
LEFT JOIN dbo.Score b
ON a.Sno=b.Sno

　　

INNER JOIN 显示交集

SELECT *
FROM dbo.Student a
INNER JOIN dbo.Score b
ON a.Sno=b.Sno

　　

right join 右表全部显示，左表没有，默认设置为NULL

SELECT *
FROM dbo.Student a
RIGHT JOIN dbo.Score b
ON a.Sno=b.Sno

　　

FULL JOIN 左右边都显示，如果对方没有值得统一设置NULL

SELECT *
FROM dbo.Student a
FULL JOIN dbo.Score b
ON a.Sno=b.Sno