$avg accumulator in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $avg accumulator do in MongoDB aggregation?

It calculates the average (mean) value of numeric values for a group of documents.

beginner

How do you use $avg to find the average score from a field named score in a collection?

Use { $group: { _id: null, averageScore: { $avg: "$score" } } } in an aggregation pipeline.

intermediate

Can $avg be used on non-numeric fields?

No, $avg only works on numeric values. Non-numeric values are ignored.

intermediate

What happens if $avg is used on an empty group of documents?

It returns null because there are no values to average.

beginner

How is $avg different from $sum in MongoDB aggregation?

$avg calculates the mean value, while $sum adds all values together.

What does the $avg accumulator return when used in a MongoDB aggregation?

A The maximum value in the group

B The total count of documents

C The sum of all values

D The average of numeric values in the group

$avg calculates the average (mean) of numeric values in the group.

Which MongoDB aggregation stage commonly uses $avg?

A <code>$match</code>

B <code>$group</code>

C <code>$project</code>

D <code>$sort</code>

$avg is used inside the $group stage to calculate averages per group.

What will $avg return if the group has no documents?

A null

B 0

C An error

D Infinity

If there are no documents in the group, $avg returns null.

Can $avg be used to average string values?

A Yes, it converts strings to numbers automatically

B Only if used with <code>$toDouble</code>

C No, it only works with numeric values

D Yes, but only if strings contain digits

$avg only works on numeric values. Strings are ignored.

Which of these is a correct use of $avg in an aggregation pipeline?

A { $group: { _id: "$category", avgPrice: { $avg: "$price" } } }

B { $match: { avgPrice: { $avg: "$price" } } }

C { $sort: { avgPrice: { $avg: "$price" } } }

D { $project: { avgPrice: { $avg: "$price" } } }

$avg is used inside $group to calculate averages per group.

Explain how the $avg accumulator works in MongoDB aggregation.

Think about how you find an average in real life.

Describe a simple example of using $avg to find the average score from a collection.

Imagine you want the average test score of all students.