今天重新溫習一遍mysql的知識
先來講講where、group by、having
group by :是將記錄中的數據，按照條件進行分組；
having：是將分組后的數據加上條件篩選，區別於where分組前的篩選。
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