http://docs.mongodb.org/manual/reference/sql-aggregation-comparison/
unwind解释 拆解数组
http://blog.nosqlfan.com/html/3648.html
几种方式做group
http://blog.csdn.net/huntzw/article/details/8669986
sql 的group by order by having 等等与mongo的对应关系,布布扣,bubuko.com
sql 的group by order by having 等等与mongo的对应关系
原文：http://www.cnblogs.com/briller/p/3868122.html