mongodbHow to count aggregated results in MongoDB?

MongoDB provides the $group aggregation operator to group documents by a specified key and apply accumulator expressions. The $group operator can be used to count the number of documents in a collection or the number of documents that match a specific criteria.

Example code

db.collection.aggregate([
    {
        $group: {
            _id: null,
            count: { $sum: 1 }
        }
    }
])

Output example

{ "_id" : null, "count" : 5 }

Code explanation

• $group: This is the aggregation operator used to group documents by a specified key.
• _id: This is the field used to specify the key to group documents by. In this example, _id is set to null to group all documents together.
• count: This is the field used to store the result of the accumulator expression.
• $sum: This is the accumulator expression used to count the number of documents.

Helpful links

• MongoDB Aggregation
• MongoDB $group

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