$sum accumulator in MongoDB

Introduction

The $sum accumulator helps add up numbers in a group. It is useful to find totals easily.

When you want to find the total sales amount from many sales records.

When you need to count how many items were sold in each category.

When you want to add up points scored by players in a game.

When you want to calculate the total expenses from a list of transactions.

Syntax

MongoDB

{ $group: { _id: <grouping_key>, total: { $sum: <expression> } } }

The $sum can add values from a field or count documents if used with 1.

The _id defines how documents are grouped before summing.

Examples

Groups documents by category and sums the amount field for each group.

MongoDB

{ $group: { _id: "$category", totalSales: { $sum: "$amount" } } }

Counts how many documents are in each status group by adding 1 for each document.

MongoDB

{ $group: { _id: "$status", count: { $sum: 1 } } }

Sample Program

This query groups sales by item name and sums the quantity sold for each item.

MongoDB

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

Important Notes

If the field used in $sum is missing or null in a document, it counts as zero.

You can use $sum inside $group or $project stages depending on your need.

Summary

$sum adds numbers or counts documents in groups.

Use it inside $group to get totals per group.

It helps quickly find totals like sales, counts, or points.

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