mongo聚合

1.db.companies.aggregate({"$project":{"companyId":1,"_id":0}})                //在查询的结果中,只出现companyId结果

2.db.companies.aggregate({"$group":{"_id":"$companyId","count":{"$sum":1}, ""}})   //这个是按照companyId进行分组,并统计分组后的个数.

结果如下:

{"_id" : ObjectId("573c46e2f9465760788df308"),"count" : 6} 其中_id就是companyId

{"_id" : ObjectId("5746cd1ff9465760788df30a"),"count" : 92}  

3.db.users_copy.aggregate({"$group":{"_id":"$companyId","count":{"$sum":1}}},{"$limit":2})  

{"$limit":2} 是只显示前两数据

4.db.users_copy.aggregate({"$group":{"_id":"$companyId","count":{"$sum":1}}},{"$skip":2}) 

{"$skip":2}是跳过前两个数据

5.db.users_copy.aggregate({"$match":{"name" : "龙钰"}},{"$group":{"_id":"$companyId","count":{"$sum":1}}})

{"$match":{"name" : "龙钰"}}    //这个是筛选条件

{"$group":{"_id":"$companyId","count":{"$sum":1}}}