$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 return type is represented by the smallest type that can store
the final value of count: integer
→
long
→ double
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:
The
$match
stage excludes documents that have ascore
value of less than or equal to80
to pass along the documents withscore
greater than80
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 calledpassing_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" } ] )