mongodb 如何使用group by

mongodb使用group by：

我们比较熟悉的group by 的sql语句select key from table groupby key,而mongoDB没提供SQL那样通过Group By就轻松实现数据库的分组功能，我们通过接口来实现的

db.collection.group({ key, reduce, initial[, keyf] [, cond] [, finalize] })

1.MonogoDB数据库中添加订单的数据

/* 0 */
{
 "_id" : ObjectId("552a330e05c27486b9b9b650"),
 "_class" : "com.mongo.model.Orders",
 "onumber" : "002",
 "date" : ISODate("2014-01-03T16:03:00Z"),
 "cname" : "zcy",
 "item" : {
   "quantity" : 1,
   "price" : 4.0,
   "pnumber" : "p002"
  }
}
 
/* 1 */
{
 "_id" : ObjectId("552a331d05c275d8590a550d"),
 "_class" : "com.mongo.model.Orders",
 "onumber" : "003",
 "date" : ISODate("2014-01-04T16:03:00Z"),
 "cname" : "zcy",
 "item" : {
   "quantity" : 10,
   "price" : 2.0,
   "pnumber" : "p001"
  }
}
 
/* 2 */
{
 "_id" : ObjectId("552a333105c2f28194045a72"),
 "_class" : "com.mongo.model.Orders",
 "onumber" : "003",
 "date" : ISODate("2014-01-04T16:03:00Z"),
 "cname" : "zcy",
 "item" : {
   "quantity" : 30,
   "price" : 4.0,
   "pnumber" : "p002"
  }
}
 
/* 3 */
{
 "_id" : ObjectId("552a333f05c2b62c01cff50e"),
 "_class" : "com.mongo.model.Orders",
 "onumber" : "004",
 "date" : ISODate("2014-01-05T16:03:00Z"),
 "cname" : "zcy",
 "item" : {
   "quantity" : 5,
   "price" : 4.0,
   "pnumber" : "p002"
  }
}

2.MongoDB实现分组并统计

1）我们要对日期和产品编码进行分组，并计算相同的产品的数量

Sql语句：

Select date, pnumber,sum(quantity) as total from orders,items group by date, pnumber

（少了两张表的关联的条件）

  MongoDB：
       db.orders.group({
          key: { date:1,'item.pnumber':1},
          initial : {"total":0},
         reduce : function Reduce(doc, out) {
        out.total+=doc.item.quantity
   } });

结果：

2）实现一天卖出了多少个产品，金额是多少，平均价格是多少

  db.orders.group({
       key: {date:1},
      initial :{"total":0,"money":0},
     reduce : function Reduce(doc, out) {
     out.total+=doc.item.quantity;
          out.money+=doc.item.quantity*doc.item.price;
        },
    finalize : function Finalize(out) {
         out.avg=out.money/out.total
         returnout;
    }
  });

结果：

3）keyf的使用

keyf 对日期进行处理并以作为key来进来分组

 db.orders.group({
   keyf: function (doc){
   return{'month':doc.date.getMonth()+1};
 },
 initial :{"total":0,"money":0},
 reduce : function Reduce(doc, out) {
    out.total+=doc.item.quantity;
          out.money+=doc.item.quantity*doc.item.price;
        
},
 finalize : function Finalize(out) {
         out.avg=out.money/out.total
         returnout;
  }
});

结果：