$count聚合运算符返回分组中文档的数量。从5.0开始支持。

语法

{ $count: { } }

$count不需要参数

使用

$count可以用于下列聚合阶段：

• $bucket
• $bucket
• $group
• $setWindowFields

在$group阶段中使用{ $sum : 1 }与$count是等价的。

举例

使用下面的命令创建cakeSales，它包含了在加利福尼亚California (CA)和华盛顿Washington (WA)的蛋糕销售记录：

db.cakeSales.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 }
] )

在$group阶段中使用

下面的例子在$group阶段中使用$count统计在cakeSales集合中每个州state的蛋糕销售数量。

在本例中：

• _id: "$state"根据state字段值对文档进行分组，分为CA和WA两个组
• $count: {}：将分组内文档数据量设置给字段countNumberOfDocumentsForState

结果如下：

{ "_id" : "CA", "countNumberOfDocumentsForState" : 3 }
{ "_id" : "WA", "countNumberOfDocumentsForState" : 3 }

在$setWindowFields阶段使用

下面的例子在$setWindowFields阶段使用$count来统计cakeSales集合所有window中文档的数量：

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            countNumberOfDocumentsForState: {
               $count: {},
               window: {
                  documents: [ "unbounded", "current" ]
               }
            }
         }
      }
   }
] )

在本例中：

• partitionBy: "$state"：根据state对集合中的文档进行分区，分别为CA和WA
• sortBy: { orderDate: 1 }根据orderDate按照从小到大对分区中的文档进行排序，最早的orderDate排在最前面
• 将window中文档数量使用$count进行汇总后赋值给countNumberOfDocumentsForState字段。

结果如下：

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "countNumberOfDocumentsForState" : 1 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "countNumberOfDocumentsForState" : 2 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "countNumberOfDocumentsForState" : 3 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "countNumberOfDocumentsForState" : 1 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "countNumberOfDocumentsForState" : 2 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "countNumberOfDocumentsForState" : 3 }