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$count (aggregation)

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Definition

$count

Returns a document that contains a count of the number of documents input to the stage.

$count has the following prototype form:

{ $count: <string> }

<string> is the name of the output field which has the count as its value. <string> must be a non-empty string, must not start with $ and must not contain the . character.

Example

A collection named scores has the following documents:

{ "_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:

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

The operation returns the following results:

{ "passing_scores" : 4 }