http://www.jianshu.com/p/c19c99a60bb7
good
http://blog.csdn.net/yutian1204/article/details/42745081
http://blog.csdn.net/u014044812/article/details/51004754
http: //www.cnblogs.com/rollenholt/p/3776923.html
sum if or sum case to speed up queries
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