$count accumulator in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $count accumulator do in MongoDB aggregation?

The $count accumulator counts the number of documents that pass through the aggregation pipeline stage and returns that count as a single number.

beginner

How do you use $count in a MongoDB aggregation pipeline?

You use $count inside a $group stage to count documents. For example: { $group: { _id: null, total: { $sum: 1 } } } counts all documents.

intermediate

Can $count be used without a $group stage in MongoDB?

No, $count is an accumulator and must be used inside a $group stage to count documents. To count documents without grouping, use the $count stage instead.

intermediate

What is the difference between the $count accumulator and the $count stage in MongoDB?

The $count accumulator is used inside $group to count documents per group. The $count stage counts all documents passing through and outputs a document with the count.

beginner

Write a simple MongoDB aggregation query using $count accumulator to count all documents.

db.collection.aggregate([{ $group: { _id: null, total: { $sum: 1 } } }])<br>This groups all documents together and counts them in the field total.

Where is the $count accumulator used in MongoDB?

A Inside a <code>$group</code> stage

B As a standalone aggregation stage

C Inside a <code>$match</code> stage

D Inside a <code>$project</code> stage

$count accumulator is used inside $group to count documents per group.

What does { $count: {} } do inside a $group stage?

A Counts the number of documents in the group

B Counts the number of fields in each document

C Counts the number of groups

D Filters documents

{ $count: {} } counts how many documents are in each group.

Which of these is a correct way to count all documents in a collection using aggregation?

A Both A and B

B Neither A nor B

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

D db.collection.aggregate([{ $count: 'total' }])

The correct way is using the $count stage: db.collection.aggregate([{ $count: 'total' }]). The $count accumulator does not exist; use $sum: 1 inside $group instead.

Can $count accumulator count documents without grouping?

A Yes, it works alone

B No, it requires a <code>$group</code> stage

C Yes, inside <code>$match</code>

D No, it only counts fields

$count accumulator must be inside $group to work.

What will this aggregation return? [{ $group: { _id: "$category", count: { $count: {} } } }]

A List of categories without counts

B Total count of all documents

C Error because <code>$count</code> is invalid

D Count of documents per category

It will cause an error because $count is not a valid accumulator. Use $sum: 1 to count documents per group.

Explain how the $count accumulator works in a MongoDB aggregation pipeline.

Think about grouping documents and counting them.

Describe the difference between the $count accumulator and the $count stage in MongoDB.

One is inside grouping, the other is a separate pipeline step.