今天重新温习一遍mysql的知识
先来讲讲where、group by、having
group by :是将记录中的数据，按照条件进行分组；
having：是将分组后的数据加上条件筛选，区别于where分组前的筛选。
http://blog.csdn.net/eclipsexys/article/details/8127252
http://blog.csdn.net/love_xsq/article/details/42417917
http://blog.csdn.net/zqtsx/article/details/41869049
https://canon4ever.gitbooks.io/c_mysql/content/mysqlzhong_de_group_by_yong_fa_jie_xi.html
http://blog.csdn.net/freshlover/article/details/8627281
http://blog.csdn.net/will130/article/details/49784625
http://blog.csdn.net/smile0198/article/details/24593135
http://www.phpstudy.net/a.php/72.html
http://zhwbqd.github.io/2015/01/06/mysql-partition-by.html