MongoDB 聚合管道（aggregate）

> db.userinfo.count()
3
> db.userinfo.find()
{ "_id" : 1, "name" : "郭大爷", "sex" : "男", "age" : "80" }
{ "_id" : 2, "name" : "郭老师", "sex" : "男", "age" : "不详" }
{ "_id" : 3, "name" : "郭少爷", "sex" : "男", "age" : "18" }

{ 
    "_id" : ObjectId("5abc960c684781cda6d38027"), 
    "name" : "郭大爷", 
    "sex" : "男", 
    "age" : 80.0, 
    "skil" : [
        "PHP", 
        "Js", 
        "MySql", 
        "Html", 
        "Css"
    ]
}
{ 
    "_id" : ObjectId("5abc960c684781cda6d38028"), 
    "name" : "郭大爷", 
    "sex" : "男", 
    "age" : 60.0, 
    "skil" : [
        "PHP", 
        "MySql"
    ]
}
{ 
    "_id" : ObjectId("5abc960c684781cda6d38029"), 
    "sex" : "女"
}
{ 
    "_id" : ObjectId("5abc960c684781cda6d3802a"), 
    "name" : "郭少爷", 
    "sex" : "男", 
    "age" : 18.0, 
    "skil" : [
        "Css", 
        "Java", 
        "Html"
    ]
}

db.userinfo.aggregate([
    { $group : {
        _id : null,
        age_sum : { $sum : "$age" }
    }}
])

{ 
    "_id" : null, 
    "age_sum" : 158.0
}