Conditional expressions ($cond, $switch) in MongoDB - Mini Project: Build & Apply

Using Conditional Expressions ($cond, $switch) in MongoDB Aggregation

📖 Scenario: You work at a bookstore that stores sales data in a MongoDB collection. You want to categorize each sale based on the number of books sold.

🎯 Goal: Build an aggregation pipeline that uses MongoDB's $cond and $switch conditional expressions to add a new field saleCategory to each document. This field will describe the sale size as 'Small', 'Medium', or 'Large' based on the number of books sold.

📋 What You'll Learn

Create a collection named sales with documents containing _id and booksSold fields.

Define a threshold variable mediumThreshold to separate small and medium sales.

Use the $cond expression to classify sales as 'Small' or 'Medium/Large'.

Use the $switch expression to further classify 'Medium/Large' sales into 'Medium' or 'Large'.

Add the final saleCategory field to each document in the aggregation result.

💡 Why This Matters

🌍 Real World

Conditional expressions in MongoDB aggregation pipelines help categorize and transform data dynamically, useful in sales analysis, reporting, and decision-making.

💼 Career

Understanding <code>$cond</code> and <code>$switch</code> is essential for data analysts and backend developers working with MongoDB to create flexible and powerful data queries.

1

Create the sales collection with sample data

Create a MongoDB collection named sales and insert these exact documents: { _id: 1, booksSold: 3 }, { _id: 2, booksSold: 7 }, { _id: 3, booksSold: 12 }.

MongoDB

// Insert the sales documents here
// Your code here

Need a hint?

Use db.sales.insertMany() with an array of documents.

2

Define the medium sales threshold

Create a variable named mediumThreshold and set it to 5 to separate small and medium sales.

MongoDB

db.sales.insertMany([
  { _id: 1, booksSold: 3 },
  { _id: 2, booksSold: 7 },
  { _id: 3, booksSold: 12 }
])

// Define the medium sales threshold
// Your code here

Need a hint?

Use const mediumThreshold = 5 to define the threshold.

3

Use $cond to classify sales as Small or Medium/Large

Write an aggregation pipeline that uses $cond to add a field saleCategory with value 'Small' if booksSold is less than mediumThreshold, otherwise 'Medium/Large'. Use $addFields stage.

MongoDB

db.sales.insertMany([
  { _id: 1, booksSold: 3 },
  { _id: 2, booksSold: 7 },
  { _id: 3, booksSold: 12 }
])

const mediumThreshold = 5

// Write aggregation pipeline with $cond to classify sales
// Your code here

Need a hint?

Use $addFields with $cond to check if booksSold is less than mediumThreshold.

4

Use $switch to refine Medium/Large classification

Extend the aggregation pipeline by adding a $set stage that uses $switch to update saleCategory: if booksSold is between mediumThreshold and 10 inclusive, set to 'Medium'; if greater than 10, set to 'Large'; otherwise keep the existing saleCategory.

MongoDB

db.sales.insertMany([
  { _id: 1, booksSold: 3 },
  { _id: 2, booksSold: 7 },
  { _id: 3, booksSold: 12 }
])

const mediumThreshold = 5

const pipeline = [
  {
    $addFields: {
      saleCategory: {
        $cond: {
          if: { $lt: ["$booksSold", mediumThreshold] },
          then: "Small",
          else: "Medium/Large"
        }
      }
    }
  }
]

// Add a $set stage with $switch to refine saleCategory
// Your code here

Need a hint?

Use $set with $switch and branches to check ranges and assign categories.