$count (aggregation)
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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.
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:
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 the following results:
{ "passing_scores" : 4 }