mysql Left join 中的笛卡尔积

表A：

select * from z_Atable

表B：

select * from z_Btable

表B中有重复数据。

A和B的內连接，即笛卡尔积：

select * from z_Atable join z_Btable

当on 条件为假时的内连接：

select * from z_Atable left join z_Btable on 0

表A作为左表，两表做左连接 left join :

select * from z_Atable a left join z_Btable b on a.Aname = b.Bname

表B作为左表，两表做左连接 left join :

select * from z_Btable a left join z_Atable b on a.Bname = b.Aname

https://www.cnblogs.com/BeginMan/p/3754322.html