MongoDB $group

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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 }

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