MongoDB 的分类汇总功能

演示代码：

let orders=[
    {cust_id:"A123",amount:500,status:"A"},
    {cust_id:"A123",amount:250,status:"A"},
    {cust_id:"B212",amount:200,status:"A"},
    {cust_id:"A123",amount:300,status:"D"},
    ]
db.orders.insertMany(orders)

//============基础汇总方式=================
db.orders.aggregate([
    {$match:{status:"A"}},//进行筛选
    {$group:{_id:'$cust_id',amount_totals:{$sum:'$amount'}}//按cust_id分类，对amount进行汇总
}])//返回结果，并显示

// =========mapReduce方式，更加通用============
db.orders.mapReduce(
    function(){emit(this.cust_id,this.amount)},//map功能，返回按cust_id分类后的amount的数组
    function(key,values){ //reduce功能，对amount数组进行汇总
        return values.reduce((total,value)=>total+value,0)
    },
    {
        query:{status:"A"},//筛选，此操作在map和reduce之前
        out:'order_totals'//输出到order_totals表
    }//
)