MongoDB Aggregation $match

Introduction

The $match stage in MongoDB's aggregation framework is used to filter documents based on specified criteria. It is similar to the find operation in MongoDB but is used within the context of an aggregation pipeline. The $match stage helps you narrow down the dataset before further processing, making your aggregation queries more efficient.

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, "category": "fruit" }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1, "category": "fruit" }
{ "_id": 3, "item": "carrot", "quantity": 7, "price": 1.5, "category": "vegetable" }
{ "_id": 4, "item": "orange", "quantity": 3, "price": 3, "category": "fruit" }
{ "_id": 5, "item": "broccoli", "quantity": 4, "price": 2.5, "category": "vegetable" }

Step 4: Using $match

1. Basic Usage of $match

To filter documents based on a specific condition, you can use the $match stage. For example, if you want to find all items in the fruit category:

db.sales.aggregate([
  { $match: { category: "fruit" } }
])

Output:

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

2. Using Comparison Operators

You can use various comparison operators in the $match stage. For example, to find items with a quantity greater than 5:

db.sales.aggregate([
  { $match: { quantity: { $gt: 5 } } } // $gt is the greater than operator
])

Output:

{ "_id": 2, "item": "banana", "quantity": 10, "price": 1, "category": "fruit" }
{ "_id": 3, "item": "carrot", "quantity": 7, "price": 1.5, "category": "vegetable" }

3. Using Logical Operators

Logical operators like $and, $or, and $not can be used to combine multiple conditions. For example, to find items that are either in the fruit category or have a price less than 2:

db.sales.aggregate([
  { 
    $match: { 
      $or: [
        { category: "fruit" },
        { price: { $lt: 2 } }
      ] 
    } 
  }
])

Output:

{ "_id": 1, "item": "apple", "quantity": 5, "price": 2, "category": "fruit" }
{ "_id": 2, "item": "banana", "quantity": 10, "price": 1, "category": "fruit" }
{ "_id": 3, "item": "carrot", "quantity": 7, "price": 1.5, "category": "vegetable" }
{ "_id": 4, "item": "orange", "quantity": 3, "price": 3, "category": "fruit" }

4. Combining $match with Other Stages

The $match stage can be combined with other aggregation stages for more complex queries. For example, if you want to find items in the fruit category and then sort them by price:

db.sales.aggregate([
  { $match: { category: "fruit" } }, // Filter by category
  { $sort: { price: 1 } } // Sort by price in ascending order
])

Output:

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

Conclusion

You have learned how to use the $match stage in MongoDB's aggregation framework to filter documents based on specified criteria. The $match stage is essential for refining your dataset and can be used in combination with other stages to create complex aggregation queries.

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