sql去重

http://blog.csdn.net/qingheshijiyuan/article/details/50410842

http://www.cnblogs.com/rainman/archive/2013/05/03/3058413.html

https://www.cnblogs.com/kingboy/p/5045713.html

http://blog.csdn.net/cngkqy/article/details/1906600