$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.

Practice

1. What does the $avg accumulator do in a MongoDB aggregation pipeline?

easy

A. Calculates the average value of numeric fields in grouped documents

B. Counts the total number of documents in a collection

C. Finds the maximum value in a group of documents

D. Sorts documents in ascending order

Solution

1. Step 1: Understand the purpose of $avg

   The $avg accumulator is designed to calculate the mean (average) of numeric values within grouped documents in an aggregation pipeline.

2. Step 2: Compare with other accumulators

   Other accumulators like $sum, $max, and $count perform different tasks, so $avg specifically calculates averages.

3. Final Answer:

   Calculates the average value of numeric fields in grouped documents -> Option A

4. Quick Check:

   $avg = average calculation [OK]

Hint: Remember: $avg means average, not count or max [OK]

Common Mistakes:

• Confusing $avg with $sum or $count
• Thinking $avg sorts data
• Assuming $avg works outside aggregation

2. Which of the following is the correct syntax to calculate the average of the field score in a MongoDB aggregation $group stage?

easy

A. { $group: { _id: null, avgScore: { $average: "$score" } } }

B. { $group: { _id: null, avgScore: { $avg: "$score" } } }

C. { $group: { _id: null, avgScore: { avg: "$score" } } }

D. { $group: { _id: null, avgScore: { $sum: "$score" } } }

Solution

1. Step 1: Identify the correct accumulator name

   The correct MongoDB accumulator for average is $avg, not $average or avg.

2. Step 2: Check the syntax structure

   The syntax requires the field name as a string with a $ prefix inside $avg, and the whole inside a $group stage with an _id.

3. Final Answer:

   { $group: { _id: null, avgScore: { $avg: "$score" } } } -> Option B

4. Quick Check:

   Correct syntax uses $avg with field string [OK]

Hint: Use $avg exactly, with field as "$fieldName" [OK]

Common Mistakes:

• Using $average instead of $avg
• Omitting quotes around field name
• Using $sum instead of $avg

3. Given the collection students with documents:

{ "name": "Alice", "score": 80 }, { "name": "Bob", "score": 90 }, { "name": "Charlie", "score": 70 }

What will be the output of this aggregation?

[{ $group: { _id: null, averageScore: { $avg: "$score" } } }]

medium

A. [{ "_id": null, "averageScore": 240 }]

B. [{ "_id": null, "averageScore": 70 }]

C. [{ "_id": null, "averageScore": 80 }]

D. [{ "_id": null, "averageScore": 90 }]

Solution

1. Step 1: Calculate the sum of scores

   Sum = 80 + 90 + 70 = 240.

2. Step 2: Calculate the average

   Average = 240 / 3 = 80.

3. Final Answer:

   [{ "_id": null, "averageScore": 80 }] -> Option C

4. Quick Check:

   Average of 80, 90, 70 = 80 [OK]

Hint: Add all scores, divide by count for average [OK]

Common Mistakes:

• Using sum instead of average
• Picking minimum or maximum score
• Forgetting to divide by number of documents

4. Consider this aggregation pipeline:

[{ $group: { _id: "$class", avgScore: { $avg: score } } }]

What is the error in this pipeline?

medium

A. Cannot use $avg inside $group

B. Missing $match stage before $group

C. The _id field cannot be "$class"

D. Field name 'score' should be a string with $ prefix: "$score"

Solution

1. Step 1: Check the field reference inside $avg

   The field name must be a string with a $ prefix, so it should be "$score", not score.

2. Step 2: Validate other parts of the pipeline

   Using "$class" as _id is valid, and $avg is allowed inside $group. $match is optional.

3. Final Answer:

   Field name 'score' should be a string with $ prefix: "$score" -> Option D

4. Quick Check:

   Field names in $avg need "$field" format [OK]

Hint: Always quote field names with $ in accumulators [OK]

Common Mistakes:

• Using unquoted field names inside $avg
• Thinking $match is mandatory before $group
• Believing _id cannot be a field name

5. You have a collection sales with documents:

{ "region": "North", "amount": 100 }, { "region": "South", "amount": 200 }, { "region": "North", "amount": 300 }, { "region": "South", "amount": 400 }

Write an aggregation pipeline to find the average sales amount per region and identify the region with the highest average sales.

hard

A. [{ $group: { _id: "$region", avgAmount: { $avg: "$amount" } } }, { $sort: { avgAmount: -1 } }, { $limit: 1 }]

B. [{ $group: { _id: "$region", avgAmount: { $sum: "$amount" } } }, { $sort: { avgAmount: 1 } }]

C. [{ $group: { _id: "$region", avgAmount: { $avg: "$amount" } } }, { $match: { avgAmount: { $gt: 300 } } }]

D. [{ $group: { _id: "$region", avgAmount: { $avg: "$amount" } } }, { $sort: { avgAmount: 1 } }, { $limit: 1 }]

Solution

1. Step 1: Group by region and calculate average

   Use $group with _id: "$region" and $avg: "$amount" to get average sales per region.

2. Step 2: Sort descending by average and limit to top

   Sort by avgAmount descending (-1) to get highest average first, then limit to 1 document.

3. Final Answer:

   [{ $group: { _id: "$region", avgAmount: { $avg: "$amount" } } }, { $sort: { avgAmount: -1 } }, { $limit: 1 }] -> Option A

4. Quick Check:

   Group, sort descending, limit 1 = highest average [OK]

Hint: Group by region, avg, sort desc, limit 1 for highest [OK]

Common Mistakes:

• Using $sum instead of $avg for average
• Sorting ascending to get highest average
• Not limiting results to top 1