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$count (accumulator operator)

Definition

$count

Returns the number of documents in a group.

The $count accumulator is available in these stages:

Note

Disambiguation

This page describes the $count aggregation accumulator. You can also use $count in these other contexts:

$count (aggregation stage), which returns the number of input documents.
$count (window function), which is used in the $setWindowFields stage to return the number of documents in a particular window.

Syntax

{ $count: { } }

The $count accumulator does not accept any parameters.

Tip

The $count accumulator is functionally equivalent to using { $sum: 1 } within the $group stage.

Examples

The examples on this page use data from the sample_mflix dataset. For details on how to load this dataset into your self-managed MongoDB deployment, see Load the Sample Dataset. If you made any modifications to the sample databases, you may need to drop and recreate the databases to run the examples on this page.

This example uses $count in the $group stage to count the number of movies in the sample_mflix.movies collection for each selected genre:

db.movies.aggregate( [
   { $unwind: "$genres" },
   { $match: { genres: { $in: [ "Drama", "Short" ] } } },
   {
      $group: {
         _id: "$genres",
         countNumberOfMoviesForGenre: {
            $count: {}
         }
      }
   },
   { $sort: { _id: 1 } }
] )

[
   { _id: 'Drama', countNumberOfMoviesForGenre: 12385 },
   { _id: 'Short', countNumberOfMoviesForGenre: 442 }
]

In the example:

$unwind: "$genres" creates a document for each value in the genres array.
$match limits the documents to the Drama and Short genres.
_id: "$genres" groups the documents by genre.
$count: {} sets the countNumberOfMoviesForGenre field to the number of movies that share the same genre.

The output shows the number of movies for Drama and Short in the countNumberOfMoviesForGenre field.

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$first