MongoDB Aggregation $group

---

Introduction

The $group stage in MongoDB’s aggregation framework is used to group documents by specific fields and perform aggregate operations like counting, summing, or averaging. This tutorial will explain how to use the $group stage effectively.

Prerequisites

• MongoDB installed and running on your machine.

• Access to mongosh and a populated database with sample data.

Step 1: Start mongosh

1. Open Terminal/Command Prompt:

   • Launch your terminal (macOS/Linux) or command prompt (Windows).

2. Start mongosh:

   • Type the following command and press Enter:

     mongosh

Step 2: Switch to the Database

Switch to the database where your collection is located.

1. Use a Database:

   use myNewDatabase

Step 3: Sample Data

Let’s assume you have a collection named sales with documents that look like this:

{ "_id": 1, "item": "apple", "quantity": 5, "price": 2 }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1 }
{ "_id": 3, "item": "apple", "quantity": 2, "price": 2 }
{ "_id": 4, "item": "orange", "quantity": 7, "price": 3 }
{ "_id": 5, "item": "banana", "quantity": 3, "price": 1 }

Step 4: Using $group

1. Basic $group Example

To group documents by the item field and calculate the total quantity sold for each item:

db.sales.aggregate([
  {
    $group: {
      _id: "$item",              // Group by 'item' field
      totalQuantity: { $sum: "$quantity" }, // Sum the 'quantity'
      totalPrice: { $sum: { $multiply: ["$quantity", "$price"] } } // Calculate total price
    }
  }
])

Output:

{ "_id": "apple", "totalQuantity": 7, "totalPrice": 14 }
{ "_id": "banana", "totalQuantity": 13, "totalPrice": 13 }
{ "_id": "orange", "totalQuantity": 7, "totalPrice": 21 }

2. Grouping with Multiple Fields

You can also group by multiple fields. For example, if you want to group by both item and price:

db.sales.aggregate([
  {
    $group: {
      _id: { item: "$item", price: "$price" }, // Group by 'item' and 'price'
      totalQuantity: { $sum: "$quantity" }
    }
  }
])

Output:

{ "_id": { "item": "apple", "price": 2 }, "totalQuantity": 7 }
{ "_id": { "item": "banana", "price": 1 }, "totalQuantity": 13 }
{ "_id": { "item": "orange", "price": 3 }, "totalQuantity": 7 }

3. Counting Documents

To count the number of documents for each item:

db.sales.aggregate([
  {
    $group: {
      _id: "$item",
      count: { $sum: 1 } // Count each document
    }
  }
])

Output:

{ "_id": "apple", "count": 2 }
{ "_id": "banana", "count": 2 }
{ "_id": "orange", "count": 1 }

4. Using $group with Other Stages

You can combine $group with other stages in the aggregation pipeline. For instance, if you want to filter documents before grouping:

db.sales.aggregate([
  { $match: { price: { $gt: 1 } } }, // Filter items with price greater than 1
  {
    $group: {
      _id: "$item",
      totalQuantity: { $sum: "$quantity" }
    }
  }
])

Output:

{ "_id": "apple", "totalQuantity": 7 }
{ "_id": "orange", "totalQuantity": 7 }

Conclusion

You have learned how to use the $group stage in MongoDB's aggregation framework to perform various aggregate operations, including summing quantities, counting documents, and grouping by multiple fields.

Feel free to reach out if you have any questions or need further guidance!

---