• JOIN: 如果表中有至少一個匹配，則返回行
• LEFT JOIN: 即使右表中沒有匹配，也從左表返回所有的行
• RIGHT JOIN: 即使左表中沒有匹配，也從右表返回所有的行
• FULL JOIN: 只要其中一個表中存在匹配，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)  

　查看數據：

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

Left Join 會顯示出左邊的全部數據，右邊如果沒有對應的值，系統默認NULL

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

INNER JOIN 顯示交集

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

right join 右表全部顯示，左表沒有，默認設置為NULL

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

FULL JOIN 左右邊都顯示，如果對方沒有值得統一設置NULL

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

SQL SERVER LEFT JOIN, INNER JOIN, RIGHT JOIN