$group stage for aggregation in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the $group stage in MongoDB aggregation?

The $group stage groups documents by a specified key and allows you to perform calculations like sums, averages, or counts on each group.

beginner

In the $group stage, what does the _id field represent?

The _id field defines the key to group documents by. Each unique value of _id creates a separate group.

beginner

How do you count the number of documents in each group using $group?

Use { $sum: 1 } inside the $group stage to add 1 for each document, effectively counting them.

intermediate

What aggregation operators can be used inside $group to calculate totals or averages?

You can use $sum to add values, $avg to calculate averages, $max for maximum, and $min for minimum values.

intermediate

Can $group stage change the shape of documents?

Yes, $group outputs one document per group with fields you define, so the shape can be very different from the original documents.

What does the _id field in $group specify?

A The name of the collection

B The output document's unique identifier

C The sorting order

D The field to group documents by

The _id field in $group defines the key used to group documents.

Which operator counts documents in each group?

A $sum: 1

B $avg

C $max

D $min

Using { $sum: 1 } adds 1 for each document, counting them.

Which operator calculates the average value in $group?

A $count

B $sum

C $avg

D $group

$avg calculates the average of numeric values in each group.

Can $group stage output documents with different fields than the input?

A Yes, it creates new documents per group

B No, it keeps the same fields

C Only if you use <code>$project</code>

D Only if you use <code>$match</code>

$group outputs one document per group with fields you define, changing the shape.

Which of these is NOT a valid accumulator operator in $group?

A $sum

B $concat

C $max

D $avg

$concat is a string operator, not an accumulator used in $group.

Explain how the $group stage works in MongoDB aggregation.

Think about how you would group items in real life, like grouping fruits by type and counting them.

Describe how to count the number of documents in each group using $group.

Counting is like adding one for each item in the group.