sql 的group by order by having 等等与mongo的对应关系

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