SQLServer 4类join查询：inner join,left join, right join, full outer join
1.from A inner join B on A.ID=B.ID :两表都有的记录才列出
A表： ID Name B表： ID Class
1 BMW 1 C1
2 Aodi 2 C2
Null Benz 4 Null
3 Null Null C4
select * from A inner join B on A.ID=B.ID:
ID Name ID Class
1 BMW 1 C1
2 Aodi 2 C2
2.from A left join B on A.ID=B.ID:
A表中所有记录列出，B中无法匹配的用Null匹配
A表： ID Name B表： ID Class
1 BMW 1 C1
2 Aodi 2 C2
Null Benz 4 Null
3 Null Null C4
select * from A left join B on A.ID=B.ID:
ID Name ID Class
1 BMW 1 C1
2 Aodi 2 C2
Null Benz Null Null
3 Null Null Null
3.from A right join B on A.ID=B.ID:
B表中所有记录列出,A中无法匹配的用Null匹配
A表： ID Name B表： ID Class
1 BMW 1 C1
2 Aodi 2 C2
Null Benz 4 Null
3 Null Null C4
select * from A right join B on A.ID=B.ID:
ID Name ID Class
1 BMW 1 C1
2 Aodi 2 C2
Null Null 4 Null
Null Null Null C4
4.from A full outer join B on A.ID=B.ID.
列出A表所有记录，B表所有记录。无法匹配的用Null替代
A表： ID Name B表： ID Class
1 BMW 1 C1
2 Aodi 2 C2
Null Benz 4 Null
3 Null Null C4
select * from A full outer join B on A.ID=B.ID.
ID Name ID Class
1 BMW 1 C1
2 Aodi 2 C2
Null Benz Null Null
3 Null Null Null
Null Null 4 Null
Null Null Null C4
其它join查询：
(1) left outer join,等价于left join.
(2)right outer join, 等价于right join.
(3) cross join, 笛卡尔乘积查询
