$sum accumulator in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sum the values of the "score" field in the aggregation.

MongoDB

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

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

Atotal

Bscore

Cvalue

Damount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field name that does not exist in the documents.

Forgetting to include the $ sign before the field name.

The $sum accumulator sums the values of the specified field. Here, we sum the "score" field.

2fill in blank

medium

Complete the code to sum the values of the "quantity" field grouped by "category".

MongoDB

db.products.aggregate([{ $group: { _id: "$"[1], totalQuantity: { $sum: "$quantity" } } }])

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

Acategory

Bprice

Cname

Dtype

Attempts:

3 left

💡 Hint

Common Mistakes

Grouping by a field that does not exist.

Using the wrong field name for grouping.

The _id field in $group specifies the grouping key. Here, we group by "category" to sum quantities per category.

3fill in blank

hard

Fix the error in the aggregation pipeline to correctly sum the "amount" field.

MongoDB

db.transactions.aggregate([{ $group: { _id: null, totalAmount: { $sum: [1] } } }])

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

A$amount

B"amount"

Camount

D$"amount"

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the field name in quotes inside $sum.

Forgetting the $ sign before the field name.

The $sum accumulator requires the field to be prefixed with a $ sign without quotes, like $amount.

4fill in blank

hard

Fill both blanks to sum the "price" field grouped by "brand".

MongoDB

db.items.aggregate([{ $group: { _id: "$"[1], totalPrice: { $sum: "$"[2] } } }])

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

Abrand

Bcategory

Cprice

Dcost

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the grouping field and the sum field.

Using incorrect field names.

Group by the "brand" field and sum the "price" field values.

5fill in blank

hard

Fill all three blanks to sum the "score" field grouped by uppercase "player" names.

MongoDB

db.games.aggregate([{ $group: { _id: { [1]: { $toUpper: "$"[2] } }, totalScore: { $sum: "$"[3] } } }])

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

A$toUpper

Bplayer

Cscore

D$toLower

Attempts:

3 left

💡 Hint

Common Mistakes

Not using an expression for _id when transforming values.

Using wrong field names for sum or group.

Use { $toUpper: "$player" } to group by uppercase player names and sum their scores.

Practice

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

easy

A. Adds values together or counts documents within groups

B. Finds the average value of a field

C. Sorts documents in ascending order

D. Filters documents based on a condition

Solution

1. Step 1: Understand the role of $sum

   The $sum accumulator adds numeric values or counts documents when used in a $group stage.

2. Step 2: Compare with other accumulators

   Unlike $avg which calculates averages, $sum only totals values or counts documents.

3. Final Answer:

   Adds values together or counts documents within groups -> Option A

4. Quick Check:

   $sum = Adds or counts [OK]

Hint: Remember: $sum totals numbers or counts docs [OK]

Common Mistakes:

• Confusing $sum with $avg
• Thinking $sum sorts data
• Assuming $sum filters documents

2. Which of the following is the correct syntax to sum the field price in a $group stage?

easy

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

B. { $group: { _id: null, total: { $sum: "$price" } } }

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

D. { $group: { _id: null, total: { sum: "$price" } } }

Solution

1. Step 1: Identify correct field reference syntax

   In MongoDB aggregation, field names must be prefixed with "$" inside expressions, so "$price" is correct.

2. Step 2: Check accumulator syntax

   The accumulator must be $sum, not sum, and the value must be a string with "$" prefix.

3. Final Answer:

   { $group: { _id: null, total: { $sum: "$price" } } } -> Option B

4. Quick Check:

   Use $sum with "$field" syntax [OK]

Hint: Use "$field" inside $sum in $group [OK]

Common Mistakes:

• Omitting the $ before field name
• Using sum instead of $sum
• Not quoting the field name

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?

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

medium

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

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

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

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

Solution

1. Step 1: Group documents by category

   Documents with category "A" are grouped together, and category "B" separately.

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

   For category "A": 10 + 20 = 30; for category "B": 5.

3. Final Answer:

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

4. Quick Check:

   Sum values per category [OK]

Hint: Sum values grouped by _id field [OK]

Common Mistakes:

• Summing without grouping
• Using null _id causing single group
• Adding only one document's value

4. Consider this aggregation pipeline:

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

What is the error in this pipeline?

medium

A. The _id field cannot be a string

B. Missing $group stage

C. Cannot use $sum inside $group

D. Field name 'value' should be quoted and prefixed with $ inside $sum

Solution

1. Step 1: Check field reference inside $sum

   The field 'value' must be referenced as "$value" (string with $ prefix) inside $sum.

2. Step 2: Confirm correct $group usage

   The $group stage is present and _id is correctly set to "$type".

3. Final Answer:

   Field name 'value' should be quoted and prefixed with $ inside $sum -> Option D

4. Quick Check:

   Use "$field" inside $sum [OK]

Hint: Always quote and prefix field with $ inside $sum [OK]

Common Mistakes:

• Using unquoted field names inside $sum
• Thinking $sum can't be used in $group
• Misunderstanding _id usage

5. You have a collection of sales documents with fields store and amount. You want to find the total sales per store but only for stores with total sales above 1000. Which aggregation pipeline correctly uses $sum and filters the results?

hard

A. [{ $match: { amount: { $gt: 1000 } } }, { $group: { _id: "$store", totalSales: { $sum: "$amount" } } }]

B. [{ $group: { _id: "$store", totalSales: { $sum: "$amount" } } }, { $match: { amount: { $gt: 1000 } } }]

C. [{ $group: { _id: "$store", totalSales: { $sum: "$amount" } } }, { $match: { totalSales: { $gt: 1000 } } }]

D. [{ $group: { _id: "$store", totalSales: { $sum: "$amount" } } }, { $match: { _id: { $gt: 1000 } } }]

Solution

1. Step 1: Group sales by store and sum amounts

   The first stage groups documents by "store" and sums the "amount" field into "totalSales".

2. Step 2: Filter groups with totalSales greater than 1000

   The second stage uses $match on the aggregated field "totalSales" to keep only stores with totals above 1000.

3. Final Answer:

   [{ $group: { _id: "$store", totalSales: { $sum: "$amount" } } }, { $match: { totalSales: { $gt: 1000 } } }] -> Option C

4. Quick Check:

   Group then filter on sum field [OK]

Hint: Group first, then filter on summed field [OK]

Common Mistakes:

• Filtering before grouping
• Filtering on original amount instead of totalSales
• Filtering on wrong field name