第一件：mysql执行顺序

一条mysql语句贯穿

[7] select

[8] [distinct] 列

[1] from 左表

[3] [left|right|full] join 右表

[2] on 连接条件

[4] where where条件

[5] group by 分组的列

[6] having having过滤条件

[9] order by 排序条件

[10] limit 数字;