mysql Left join 中的笛卡爾積

表A：

select * from z_Atable

表B：

select * from z_Btable

表B中有重複數據。

A和B的內連接，即笛卡爾積：

select * from z_Atable join z_Btable

當on 條件爲假時的內連接：

select * from z_Atable left join z_Btable on 0

表A作爲左表，兩表做左連接 left join :

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

表B作爲左表，兩表做左連接 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