mongo

https://www.cnblogs.com/zhoujie/p/mongo1.html 聚合用法、
https://www.cnblogs.com/luoaz/p/4691639.html mongo数据库集合的使用方法
https://blog.youkuaiyun.com/qq_39263663/article/details/80459833 mongodb的$用法
https://blog.youkuaiyun.com/qq_18769269/article/details/81143265 实例
https://www.jianshu.com/p/10605a604c8c 多条件聚合语句
https://blog.youkuaiyun.com/xb12369/article/details/51499504 多条件聚合实例