$group stage for aggregation in MongoDB - Step-by-Step Execution

Concept Flow - $group stage for aggregation

Input Documents

↓

Apply $group Stage

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Group by _id Key

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Aggregate Fields (sum, avg, etc.)

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Output Grouped Documents

The $group stage collects input documents, groups them by a specified key (_id), and computes aggregate values for each group.

Execution Sample

MongoDB

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

Groups sales documents by item and sums the quantity for each item.

Execution Table

Step	Input Document	Group Key (_id)	Aggregation Action	Group State After Step
1	{ item: "apple", quantity: 5 }	"apple"	Create group 'apple', sum=5	{ apple: 5 }
2	{ item: "banana", quantity: 3 }	"banana"	Create group 'banana', sum=3	{ apple: 5, banana: 3 }
3	{ item: "apple", quantity: 2 }	"apple"	Add 2 to sum of 'apple'	{ apple: 7, banana: 3 }
4	{ item: "banana", quantity: 4 }	"banana"	Add 4 to sum of 'banana'	{ apple: 7, banana: 7 }
5	{ item: "orange", quantity: 1 }	"orange"	Create group 'orange', sum=1	{ apple: 7, banana: 7, orange: 1 }
6	No more documents	N/A	Aggregation complete	{ apple: 7, banana: 7, orange: 1 }

💡 All input documents processed; aggregation groups finalized.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Group 'apple' sum	0	5	5	7	7	7	7
Group 'banana' sum	0	0	3	3	7	7	7
Group 'orange' sum	0	0	0	0	0	1	1

Key Moments - 3 Insights

Why does the $group stage use _id as the grouping key?

The _id field defines how documents are grouped. Each unique _id value creates a separate group. This is shown in the execution_table where each input document's item field becomes the _id key.

What happens when a new group key appears in the input documents?

A new group is created with initial aggregation values. For example, when 'orange' appears at step 5, a new group is created with sum starting at the quantity of that document.

How does the aggregation update existing groups?

When a document's group key matches an existing group, aggregation operators like $sum add to the current total. See step 3 where 'apple' sum increases from 5 to 7.

Visual Quiz - 3 Questions

Look at the execution_table, what is the sum for group 'banana' after step 4?

A 7

B 3

C 4

D 0

💡 Hint

Check the 'Group State After Step' column at step 4 for 'banana'.

At which step does the group 'orange' get created?

A Step 2

B Step 3

C Step 5

D Step 6

💡 Hint

Look for when 'orange' first appears in the 'Group Key (_id)' column.

If the third document had quantity 4 instead of 2, what would be the final sum for 'apple'?

A 7

B 9

C 11

D 5

💡 Hint

Add the changed quantity to the previous sum for 'apple' from the variable_tracker.

Concept Snapshot

$group stage syntax:
{ $group: { _id: <grouping_key>, <field>: { <accumulator>: <expression> } } }
Groups documents by _id and applies accumulators like $sum, $avg.
Each unique _id value creates one output document.
Useful for summarizing data by categories.

Full Transcript

The $group stage in MongoDB aggregation groups input documents by a specified key called _id. Each unique _id value forms a group. For each group, accumulator operators like $sum add values from documents in that group. The execution table shows step-by-step how documents are processed: new groups are created when a new _id appears, and existing groups update their totals. Variables track sums for each group after each step. Key moments clarify why _id is used, how new groups form, and how aggregation updates work. The visual quiz tests understanding of sums at steps, group creation timing, and effects of changing input values. The snapshot summarizes syntax and behavior for quick reference.