目录
场景1：left join on a.xx = b.xx and a.xx2 = 'aa'
场景2：left join on a.xx = b.xx and b.xx2 = 'aa'
场景3：left join on a.xx = b.xx where b.xx2 = 'aa'
场景4：inner join on a.xx = b.xx where a.xx2 = 'aa'
场景5：inner join on a.xx = b.xx and a.xx2 = 'aa'
场景6：inner join on a.xx = b.xx and b.xx2 = 'aa'
总结
场景1：left join on a.xx = b.xx and a.xx2 = 'aa'
场景2：left join on a.xx = b.xx and b.xx2 = 'aa'
场景3：left join on a.xx = b.xx where b.xx2 = 'aa'
场景4：inner join on a.xx = b.xx where a.xx2 = 'aa'
场景5：inner join on a.xx = b.xx and a.xx2 = 'aa'
场景6：inner join on a.xx = b.xx and b.xx2 = 'aa'
总结
1、where 条件可以理解为，关联后，数据作为一个整体的过滤条件
2、on 后面的and 条件为关联条件，如果是left join，则不对主表记录行数产生影响；如果是inner join则，效果等同于where条件
