MongoDB $group - group by 分组使用

1.操作的表 （Collection） 的数据

{ " _id" : 1, " item" : "abc", " price" : 10, " quantity" : 2, " date" : ISODate("2014-03-01T08:00:00Z") }

{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-03-01T09:00:00Z") }

{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-03-15T09:00:00Z") }

{ "_id" : 4, "item" : "xyz", "price" : 5, "quantity" : 20, "date" : ISODate("2014-04-04T11:21:39.736Z") }

{ "_id" : 5, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-04-04T21:23:13.331Z") }

---

2.group by year&month&day - 通过年月日来分组统计

db.sales.aggregate(
  [
      {

        $group : {

         _id :{ month: { $month: "$date" }, day: { $dayOfMonth: "$date" }, year: { $year: "$date" }}, #根据年月日统计

          totalPrice: { $sum: { $multiply: [ "$price", "$quantity" ] } }, #totalPrice = (price_1 * quantity_1) + (price_2 * quantity_2) + . . .  

          averageQuantity: { $avg: "$quantity" }, #求平均数

          count: { $sum: 1 } #记录数

        }
      }
  ])

---

3.执行结果

{ "_id" : { "month" : 3, "day" : 15, "year" : 2014 }, "totalPrice" : 50, "averageQuantity" : 10, "count" : 1 }

{ "_id" : { "month" : 4, "day" : 4, "year" : 2014 }, "totalPrice" : 200, "averageQuantity" : 15, "count" : 2 }

{ "_id" : { "month" : 3, "day" : 1, "year" : 2014 }, "totalPrice" : 40, "averageQuantity" : 1.5, "count" : 2 }