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Database Manual

Aggregation Operations

Reference

Stages

$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).

Compatibility

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

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

MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB

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.

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:

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.
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" }
] )

The C# examples on this page use the sample_mflix database from the Atlas sample datasets. To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started in the MongoDB .NET/C# Driver documentation.

The following Movie class models the documents in the sample_mflix.movies collection:

public class Movie
{
    public string Id { get; set; }
    public int Runtime { get; set; }
    
    public string Title { get; set; }
    public string Rated { get; set; }
    public List<string> Genres { get; set; }
    public string Plot { get; set; }
    
    public ImdbData Imdb { get; set; }
    public int Year { get; set; }
    public int Index { get; set; }
    
    public string[] Comments { get; set; }
   
    [BsonElement("lastupdated")]
    public DateTime LastUpdated { get; set; }
}

To use the MongoDB .NET/C# driver to add a $count stage to an aggregation pipeline, call the Count() method on a PipelineDefinition object.

The following example creates a pipeline stage that counts the number of input documents and returns a document with the count as its value:

var pipeline = new EmptyPipelineDefinition<Movie>()
    .Count();

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