$count (aggregation)
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Definition
-
$count
-
Returns a document that contains a count of the number of documents input to the stage.
$count
has the following prototype form:<string>
is the name of the output field which has the count as its value.<string>
must be a non-empty string, must not start with$
and must not contain the.
character.See also
Behavior
The $count
stage is equivalent to the following $group
+ $project
sequence:
where myCount
would be the output field that contains the count. You can specify another name for the output field.
Example
A collection named scores
has the following documents:
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
.
The operation returns the following results: