MongoDB 聚合管道（aggregate）

1、聚合函数

• 查询总数 .count()

> 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" }

2、aggregate() 方法

我们先插入一些测试数据

{
    "_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"
    ]
}

• 求和

查询一下所有人年龄的总和

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

结果

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

• 平均值

• 最小值

• 最大值

• 获取第一个文档

• 获取最后一个文档

原文地址：https://www.cnblogs.com/handsCool/p/8669811.html