首先有下面的数据库
A 表
id name
1 hello
2 world
B 表
aid book flag
1 mysql 0
1 android 0
2 mysql 1
2 mysql 0
2 android 0
下面开始进行几种查询
1.inner join
select * from a,b where a.id=baid; select * from a inner join b on a.id=b.aid;
两种查询是等价的
2.左右连接
A left join B 的连接的记录数与A表的记录数同 A right join B 的连接的记录数与B表的记录数同 A left join B 等价B right join A
select * from A left join B on A.id = B.aid
3.查处不连接后不重复的数据
select * from A left join B on A.id = B.aid select *,count(distict A.id) from A left join B on A.id = B.aid group by A.id