$count (aggregation)¶

Definition定义¶

$count¶

New in version 3.4.版本3.4中的新功能。

Passes a document to the next stage that contains a count of the number of documents input to the stage.将文档传递到下一个阶段，该阶段包含输入到该阶段的文档数计数。

$count has the following prototype form:具有以下原型形式：

{ $count: <string> }

<string> is the name of the output field which has the count as its value. 是以计数作为其值的输出字段的名称。<string> must be a non-empty string, must not start with $ and must not contain the . character.必须是非空字符串，不得以$开头，且不得包含.字符。

See also参阅

• db.collection.countDocuments()
• $collStats
• db.collection.estimatedDocumentCount()
• count
• db.collection.count()

Behavior行为¶

The $count stage is equivalent to the following $group + $project sequence:$count阶段相当于以下$group+$project序列：

db.collection.aggregate( [
   { $group: { _id: null, myCount: { $sum: 1 } } },
   { $project: { _id: 0 } }
] )

where myCount would be the output field that contains the count. 其中myCount是包含计数的输出字段。You can specify another name for the output field.可以为输出字段指定另一个名称。

Example示例¶

A collection named scores has the following documents:名为scores的集合包含以下文档：

{ "_id" : 1, "subject" : "History", "score" : 88 }
{ "_id" : 2, "subject" : "History", "score" : 92 }
{ "_id" : 3, "subject" : "History", "score" : 97 }
{ "_id" : 4, "subject" : "History", "score" : 71 }
{ "_id" : 5, "subject" : "History", "score" : 79 }
{ "_id" : 6, "subject" : "History", "score" : 83 }

The following aggregation operation has two stages:以下聚合操作分为两个阶段：

1. The $match stage excludes documents that have a score value of less than or equal to 80 to pass along the documents with score greater than 80 to the next stage.$match阶段不包括得score小于或等于80的文档，以便将score大于80的文档传递到下一阶段。
2. The $count stage returns a count of the remaining documents in the aggregation pipeline and assigns the value to a field called passing_scores.$count阶段返回聚合管道中剩余文档的计数，并将该值分配给名为passing_scores的字段。

db.scores.aggregate(
  [
    {
      $match: {
        score: {
          $gt: 80
        }
      }
    },
    {
      $count: "passing_scores"
    }
  ]
)

The operation returns the following results:操作返回以下结果：

{ "passing_scores" : 4 }