转载链接：https://blog.csdn.net/qq_42721964/article/details/86545189
统计：
> db.emp.aggregate([{$match: {"age": {$gt: 18, $lt: 38}}},{$count: "num"}])
{ "num" : 3 }
> db.emp.count({"age": {$gt: 18, $lt: 38}})
3
> db.emp.count({"age": {$gt: 18, $lt: 38}, "_id": 13})
1