$sum accumulator in MongoDB - Step-by-Step Execution

Concept Flow - $sum accumulator

Start with empty sum = 0

↓

Read each document's value

↓

Add value to sum

↓

Repeat for all documents

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Output total sum

The $sum accumulator adds up values from each document in a group and outputs the total sum.

Execution Sample

MongoDB

db.sales.aggregate([
  { $group: { _id: null, totalSales: { $sum: "$amount" } } }
])

This query sums the 'amount' field from all documents in the 'sales' collection.

Execution Table

Step	Document amount	Current sum	Action	Sum after action
1	100	0	Add 100 to sum	100
2	200	100	Add 200 to sum	300
3	50	300	Add 50 to sum	350
4	150	350	Add 150 to sum	500
5	-	500	No more documents	Final sum = 500

💡 All documents processed, sum accumulator outputs 500

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
sum	0	100	300	350	500	500

Key Moments - 2 Insights

Why does the sum start at 0 before adding any values?

The sum accumulator initializes at 0 to correctly add values from documents. See execution_table row 1 where sum is 0 before adding the first amount.

What happens if a document's amount field is missing or null?

If the amount is missing or null, $sum treats it as 0 and does not change the sum. This is why each addition explicitly adds the document's amount value if present.

Visual Quiz - 3 Questions

Look at the execution table, what is the sum after processing the second document?

A 100

B 200

C 300

D 350

💡 Hint

Check the 'Sum after action' column at step 2 in the execution_table.

At which step does the sum reach 500?

A Step 3

B Step 4

C Step 5

D Step 2

💡 Hint

Look at the 'Sum after action' column and find when it first shows 500.

If the third document's amount was 0 instead of 50, what would the sum be after step 3?

A 300

B 350

C 250

D 500

💡 Hint

Refer to variable_tracker and consider how sum changes when adding 0 instead of 50.

Concept Snapshot

$sum accumulator in MongoDB groups documents and adds values.
Starts at 0, adds each document's field value.
Ignores missing or null values as 0.
Outputs total sum after processing all documents.

Full Transcript

The $sum accumulator in MongoDB is used to add up values from documents in a group. It starts with a sum of zero. For each document, it reads the value of the specified field and adds it to the sum. This continues until all documents are processed. If a document does not have the field or it is null, it adds zero. The final output is the total sum of all values. For example, if documents have amounts 100, 200, 50, and 150, the sum accumulates as 100, then 300, then 350, and finally 500. This process helps quickly calculate totals in aggregation queries.