Why aggregation operators matter in MongoDB - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the total number of documents in the collection.

MongoDB

db.sales.aggregate([{ $group: { _id: null, total: { $[1]: 1 } } }])

Drag options to blanks, or click blank then click option'

Asum

Bcount

Cavg

Dmax

Attempts:

3 left

💡 Hint

Common Mistakes

Using $count inside $group is invalid.

Using $avg or $max will not count documents.

The $sum operator adds values together. Here, it counts each document by adding 1 for each.

2fill in blank

medium

Complete the code to find the average price of all products.

MongoDB

db.products.aggregate([{ $group: { _id: null, averagePrice: { $[1]: "$price" } } }])

Drag options to blanks, or click blank then click option'

Amax

Bsum

Cmin

Davg

Attempts:

3 left

💡 Hint

Common Mistakes

Using $sum will give total, not average.

Using $min or $max will give smallest or largest value, not average.

The $avg operator calculates the average of the specified field values.

3fill in blank

hard

Fix the error in the aggregation to find the maximum score.

MongoDB

db.scores.aggregate([{ $group: { _id: null, maxScore: { $[1]: "$score" } } }])

Drag options to blanks, or click blank then click option'

Amax_value

BmaxScore

Cmax

Dmaximum

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'maximum' or 'maxScore' as operator names causes errors.

Misspelling the operator leads to failure.

The correct operator to find the maximum value is $max. Other options are invalid.

4fill in blank

hard

Fill both blanks to calculate the total sales and average quantity.

MongoDB

db.orders.aggregate([{ $group: { _id: "$product", totalSales: { $[1]: "$amount" }, averageQty: { $[2]: "$quantity" } } }])

Drag options to blanks, or click blank then click option'

Asum

Bavg

Cmax

Dmin

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up $sum and $avg operators.

Using $max or $min instead of sum or average.

$sum adds all amounts for total sales, and $avg calculates the average quantity.

5fill in blank

hard

Fill all three blanks to group by category, count items, and find max price.

MongoDB

db.items.aggregate([{ $group: { _id: "$category", count: { $[1]: 1 }, maxPrice: { $[2]: "$price" }, minPrice: { $[3]: "$price" } } }])

Drag options to blanks, or click blank then click option'

Asum

Bmax

Cmin

Davg

Attempts:

3 left

💡 Hint

Common Mistakes

Using $avg instead of $sum for counting.

Confusing $max and $min operators.

$sum counts items by adding 1 each, $max finds the highest price, and $min finds the lowest price.

Practice

1. What is the main purpose of aggregation operators in MongoDB?

easy

A. To summarize and analyze data quickly

B. To insert new documents into a collection

C. To delete documents from a collection

D. To create indexes on collections

Solution

1. Step 1: Understand aggregation operators

   Aggregation operators are used to process data and return computed results like sums, averages, and counts.

2. Step 2: Compare with other operations

   Inserting, deleting, and indexing are different operations unrelated to aggregation.

3. Final Answer:

   To summarize and analyze data quickly -> Option A

4. Quick Check:

   Aggregation = Summarize data [OK]

Hint: Aggregation operators summarize data fast [OK]

Common Mistakes:

• Confusing aggregation with data insertion
• Thinking aggregation deletes data
• Mixing aggregation with indexing

2. Which of the following is the correct syntax to use the $sum aggregation operator in a MongoDB pipeline?

easy

A. { $group: { _id: "$amount", total: $sum } }

B. { $sum: { $group: { total: "$amount" } } }

C. { $group: { _id: null, total: { $sum: "$amount" } } }

D. { $group: { total: { sum: "$amount" } } }

Solution

1. Step 1: Review $group stage syntax

   The $group stage groups documents and applies accumulators like $sum to fields.

2. Step 2: Check correct $sum usage

   $sum must be inside $group with a field path as "$fieldName" and an _id for grouping.

3. Final Answer:

   { $group: { _id: null, total: { $sum: "$amount" } } } -> Option C

4. Quick Check:

   $sum inside $group with field path [OK]

Hint: Use $sum inside $group with field path in quotes [OK]

Common Mistakes:

• Placing $sum outside $group
• Missing quotes around field name
• Incorrect _id usage in $group

3. Given the collection documents: [{"category": "A", "value": 10}, {"category": "A", "value": 20}, {"category": "B", "value": 5}], what will be the output of this aggregation pipeline?

[{ $group: { _id: "$category", totalValue: { $sum: "$value" } } }]

medium

A. [{ "_id": "A", "totalValue": 10 }, { "_id": "B", "totalValue": 5 }]

B. [{ "_id": "A", "totalValue": 30 }, { "_id": "B", "totalValue": 5 }]

C. [{ "_id": null, "totalValue": 35 }]

D. [{ "_id": "A", "totalValue": 20 }, { "_id": "B", "totalValue": 5 }]

Solution

1. Step 1: Group documents by category

   The $group stage groups documents by the "category" field, so we get groups for "A" and "B".

2. Step 2: Sum the "value" fields per group

   For category "A", sum is 10 + 20 = 30; for "B", sum is 5.

3. Final Answer:

   [{ "_id": "A", "totalValue": 30 }, { "_id": "B", "totalValue": 5 }] -> Option B

4. Quick Check:

   Sum values grouped by category [OK]

Hint: Sum values grouped by _id field [OK]

Common Mistakes:

• Forgetting to group by category
• Summing all values without grouping
• Using null as _id incorrectly

4. Identify the error in this aggregation pipeline:

[{ $group: { _id: "$category", total: { $sum: value } } }]

medium

A. Missing quotes around the field name "value" inside $sum

B. Using $sum inside $group is not allowed

C. The _id field cannot be "$category"

D. The pipeline must start with $match stage

Solution

1. Step 1: Check $sum syntax

   The field name inside $sum must be a string with quotes, like "$value".

2. Step 2: Validate other parts

   Using $sum inside $group and _id as "$category" are correct; $match is optional.

3. Final Answer:

   Missing quotes around the field name "value" inside $sum -> Option A

4. Quick Check:

   Field names in $sum need quotes [OK]

Hint: Always quote field names in aggregation operators [OK]

Common Mistakes:

• Omitting quotes around field names
• Thinking $sum can't be used in $group
• Assuming $match is mandatory first stage

5. You want to find the average "score" for each "team" but only for scores greater than 50. Which aggregation pipeline correctly achieves this?

hard

A. [{ $match: { team: { $gt: 50 } } }, { $group: { _id: "$team", avgScore: { $avg: "$score" } } }]

B. [{ $group: { _id: "$team", avgScore: { $avg: { $gt: ["$score", 50] } } } }]

C. [{ $group: { _id: "$team", avgScore: { $avg: "$score" } } }, { $match: { avgScore: { $gt: 50 } } }]

D. [{ $match: { score: { $gt: 50 } } }, { $group: { _id: "$team", avgScore: { $avg: "$score" } } }]

Solution

1. Step 1: Filter scores greater than 50

   The $match stage filters documents where "score" is greater than 50 before grouping.

2. Step 2: Group by "team" and calculate average

   The $group stage groups by "team" and calculates average "score" using $avg.

3. Step 3: Verify other options

   [{ $group: { _id: "$team", avgScore: { $avg: { $gt: ["$score", 50] } } } }] misuses $avg with $gt; [{ $group: { _id: "$team", avgScore: { $avg: "$score" } } }, { $match: { avgScore: { $gt: 50 } } }] filters after averaging, which is incorrect; [{ $match: { team: { $gt: 50 } } }, { $group: { _id: "$team", avgScore: { $avg: "$score" } } }] filters on "team" field incorrectly.

4. Final Answer:

   [{ $match: { score: { $gt: 50 } } }, { $group: { _id: "$team", avgScore: { $avg: "$score" } } }] -> Option D

5. Quick Check:

   Filter first, then group and average [OK]

Hint: Filter with $match before grouping for correct averages [OK]

Common Mistakes:

• Filtering after grouping instead of before
• Misusing $avg with conditions inside
• Filtering wrong fields in $match