golang+mongodb 怎么樣實現聚合查詢，按字段分組並查找出相應字段

集合sales的定義如下：

{ "_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") }

聚合函數如下：

db.sales.aggregate(
   [
      {
        $group : {
           _id : { month: { $month: "$date" }, day: { $dayOfMonth: "$date" }, year: { $year: "$date" } },
           totalPrice: { $sum: { $multiply: [ "$price", "$quantity" ] } },
           averageQuantity: { $avg: "$quantity" },
           count: { $sum: 1 }
        }
      }
   ]
)

{ "_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 }

或者在mongochef中的json view如下：

{ 
    "_id" : {
        "month" : NumberInt(4), 
        "day" : NumberInt(4), 
        "year" : NumberInt(2014)
    }, 
    "totalPrice" : 200.0, 
    "averageQuantity" : 15.0, 
    "count" : NumberInt(2)
}
{ 
    "_id" : {
        "month" : NumberInt(3), 
        "day" : NumberInt(15), 
        "year" : NumberInt(2014)
    }, 
    "totalPrice" : 50.0, 
    "averageQuantity" : 10.0, 
    "count" : NumberInt(1)
}
{ 
    "_id" : {
        "month" : NumberInt(3), 
        "day" : NumberInt(1), 
        "year" : NumberInt(2014)
    }, 
    "totalPrice" : 40.0, 
    "averageQuantity" : 1.5, 
    "count" : NumberInt(2)
}

實例說明：

1、

 

 _id : { month: { $month: "$date" }, day: { $dayOfMonth: "$date" }, year: { $year: "$date" } },

此處指定的“_id” 必需且唯一，意為根據所列的域（字段）進行分組。

如果指定： _id : null

則意為對所有行進行分組統計，

例子如下：

db.sales.aggregate(
   [
      {
        $group : {
           _id : null,
           totalPrice: { $sum: { $multiply: [ "$price", "$quantity" ] } },
           averageQuantity: { $avg: "$quantity" },
           count: { $sum: 1 }
        }
      }
   ]
)

查詢結果如下：

 

{ "_id" : null, "totalPrice" : 290, "averageQuantity" : 8.6, "count" : 5 }

 

 

2、

$sum:1

表示每條記錄按照數量1計數，

如果設為2，則每條記錄（文檔、每行）按照數量2計數，然后 總數=計數*行數（記錄數*每條記錄的計數）。

3、

 

$month: "$date" 

 

 

根據日期字段取該日期對應的月份。