order by:

　　用来对数据库的一组数据进行排序

　　 desc：降序

　　 asc：升序

group by：

　　“By”指定的规则对数据进行分组，所谓的分组就是将一个“数据集”划分成若干个“小区域”，然后针对若干个“小区域”进行数据处理。

原始表：

1、order by

　　（1）降序

select * from aaa order by num desc

　　（2）升序

select * from aaa order by num asc

2、group by分组

　　(1)分类汇总

select lei,sum(num) as num from aaa group by lei

(2)和order by连用

select lei,sum(num) as num from aaa group by lei order by num desc

(3)按几个条件分组，如下按照lei和info进行分组

select lei,sum(num) as num, info from aaa group by lei, info

(4)和聚合函数一起使用

select lei,avg(num) as from aaa group by lei

select lei, count(num) as num from aaa group by lei

　查出每个lei有几条记录:

常见的聚合函数：