mongodb mapReduce

Posted 2022-12-02

mapReduce随着大数据概念而流行，从功能上来说，相当于RDBMS的group操作，mapReduce真正的强项在于分布式

1.用mapReduce计算每个栏目的库存总量 

var map = function()
                     emit(this.cat_id,this.goods_number);
                    
var reduce = function(cat_id,numbers)
                                      return Array.sum(numbers);
                                     
db.goods.mapReduce(map,reduce,query:,out:res)

2.用mapReduce计算每个栏目下的商品的平均价格 

var map = function()
  emit(this.cat_id,this.shop_price);


var reduce = function(cat_id,values)
  return Array.avg(values);


db.goods.mapReduce(map,reduce,query:,out:res)

3.计算地震数据，统计某一个经纬度地震的次数

导入国家地震数据

用mapReduce进行统计

var map = function()
   if(this.jing < 0 || this.wei < 0)
      return;
   
   var j = Math.floor(this.jing / 5) *5;
   var w = Math.floor(this.wei / 5) * 5;
   var block = j + : + w

   emit(block,1)


var reduce = function(block,values)
   return Array.sum(values);


db.dz.mapReduce(map,reduce,out:res)

统计平均深度 

var map = function()
    if(this.jing < 0 || this.wei < 0)
        return;
    
   var j = Math.floor(this.jing / 5) *5;
   var w = Math.floor(this.wei / 5) * 5;
   var block = j + : + w

   emit(block,this.lev)


var reduce = function(block,values)
   return Array.avg(values);


db.dz.mapReduce(map,reduce,out:res)

db.res.find().sort(value:-1)