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
$countaggregation pipeline stage. For the$countaggregation accumulator, see$count (aggregation accumulator).$counthas 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:
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
$matchstage excludes documents that have ascorevalue of less than or equal to80to pass along the documents withscoregreater than80to the next stage. -
The
$countstage 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 } |