$count (aggregation)

https://www.mongodb.com/docs/manual/reference/operator/aggregation/count/

 

$count (aggregation)

 

Definition

 

$count

 

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).

$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.

TIP

See also:

• db.collection.countDocuments()

• $collStats

• db.collection.estimatedDocumentCount()

• count

• db.collection.count()

Behavior

 

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

db.collection.aggregate( [

{ $group: { _id: null, myCount: { $sum: 1 } } },

{ $project: { _id: 0 } }

] )

 

where myCount would be the output field that contains the count. You can specify another name for the output field.

TIP

See also:

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

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 }