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Aggregation Pipeline Stages

$count (aggregation)

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  • Definition
  • Compatibility
  • Syntax
  • Behavior
  • Examples
  • Learn More

Definition

$count

New in version 3.4.

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

Note

Disambiguation

This page describes the $count aggregation pipeline stage. For the $count aggregation accumulator, see $count (aggregation accumulator).

Compatibility

You can use $count for deployments hosted in the following environments:

  • MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Syntax

$count has the following syntax:

{ $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.

Behavior

The $count stage is equivalent to the following $group and $project sequence:

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

myCount is the output field that stores the count. You can specify another name for the output field.

If the input dataset is empty, $count doesn't return a result.

Tip

See also:

db.collection.countDocuments() which wraps the $group aggregation stage with a $sum expression.

Examples

Create a collection named scores with these documents:

db.scores.insertMany( [
   { "_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 this result:

{ "passing_scores" : 4 }

If the input dataset is empty, $count doesn't return a result. The following example doesn't return a result because there are no documents with scores greater than 99:

db.scores.aggregate( [
   { $match: { score: { $gt: 99 } } },
   { $count: "high_scores" }
] )

Learn More

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