order by 后面可以跟多个字段进行排序
用A1代表第一个字段，A2代表第二个字段
一、order by A1 , A2 desc 指的是用A1升序A2降序
二、order by A1 asc , A2 desc 指的是用A1升序A2降序
三、order by A1 desc , A2 desc 指的是用A1降序A2降序
