MongoDB Aggregation $group

MongoDB Aggregation $group 📊

It groups documents by a key and performs calculations (aggregations) on each group—similar to GROUP BY in SQL.

What does $group do?

• Combines multiple documents into one document per group

• Uses accumulator operators like $sum, $avg, $min, $max

• Requires an _id field (grouping key)

Basic Syntax

📌 _id defines how documents are grouped

1️⃣ Group by a Field (Most Common)

Example: Count students per course

Output

2️⃣ Group + Average, Min, Max

Example: Average age per course

3️⃣ Group All Documents Together

Use _id: null

📌 Useful for overall totals

4️⃣ Group with $match (Best Practice 👍)

Example: Count BA students above age 20

⚡ $match first = better performance

5️⃣ Group by Multiple Fields

Example: Group by course + gender

6️⃣ $group with $sum on a Field

Example: Total marks per course

7️⃣ $first and $last

📌 Usually combined with $sort

Common $group Accumulators

SQL vs MongoDB $group

SQL

MongoDB

Common Mistakes ⚠️

• Forgetting _id in $group

• Using $group before $match

• Expecting original document fields after grouping

Quick Recap 🧠