order by a desc,b desc与order by a,b desc的区别
表为：
通过下面sql看出 order by 默认是asc 。相当于select * from order_table order by id1 asc
通过下面两个图可知，
select * from order_table order by id1,id2 desc不等于 select * from order_table order by id1 desc,id2 desc.
select * from order_table order by id1,id2 desc等于select * from order_table order by id1 asc,id2 desc
通过上图和下图可知，select * from order_table order by id1 desc,id2 desc不等于 select * from order_table order by id2 desc,id1 desc.谁在前优先级高，先排序id1，id1相同的情况下再排序id2