Arithmetic expressions ($add, $multiply, $divide) in MongoDB - Mini Project: Build & Apply

Using Arithmetic Expressions in MongoDB Aggregation

📖 Scenario: You work in a small store that keeps product sales data in a MongoDB collection. You want to calculate the total revenue for each product by multiplying the quantity sold by the price per unit. You also want to add a fixed tax amount to the total revenue and then divide the final amount by 2 to find the average revenue over two months.

🎯 Goal: Build a MongoDB aggregation pipeline that uses the arithmetic expressions $add, $multiply, and $divide to calculate the average revenue including tax for each product.

📋 What You'll Learn

Create a collection named sales with documents containing product, quantity, and price fields.

Add a fixed tax amount variable named tax with the value 5.

Use $multiply to calculate total revenue by multiplying quantity and price.

Use $add to add the tax to the total revenue.

Use $divide to divide the sum by 2 to get the average revenue.

Output the product name and the calculated average revenue as averageRevenue.

💡 Why This Matters

🌍 Real World

Stores and businesses often need to calculate revenues, taxes, and averages from sales data stored in databases like MongoDB.

💼 Career

Understanding MongoDB aggregation and arithmetic expressions is essential for data analysts and backend developers working with NoSQL databases.

1

Create the sales collection with product data

Create a MongoDB collection named sales with these exact documents: { product: "Pen", quantity: 10, price: 2 }, { product: "Notebook", quantity: 5, price: 4 }, and { product: "Eraser", quantity: 20, price: 1 }.

MongoDB

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

Hint

Use db.sales.insertMany([...]) to add multiple documents at once.

2

Add a tax variable for the fixed tax amount

Create a variable named tax and set it to the number 5 to represent the fixed tax amount.

MongoDB

db.sales.insertMany([
  { product: "Pen", quantity: 10, price: 2 },
  { product: "Notebook", quantity: 5, price: 4 },
  { product: "Eraser", quantity: 20, price: 1 }
])

// Create the tax variable below
// Your code here

Hint

Use const tax = 5 to create a fixed tax variable.

3

Calculate total revenue and add tax using $multiply and $add

Write a MongoDB aggregation pipeline that uses $multiply to multiply quantity and price to get total revenue, then uses $add to add the tax variable to this total. Use $project to output product and the calculated totalWithTax.

MongoDB

db.sales.insertMany([
  { product: "Pen", quantity: 10, price: 2 },
  { product: "Notebook", quantity: 5, price: 4 },
  { product: "Eraser", quantity: 20, price: 1 }
])

const tax = 5

// Write the aggregation pipeline below
// Your code here

Hint

Use $project to create a new field totalWithTax that adds tax to the product of quantity and price.

4

Divide the total with tax by 2 using $divide to get average revenue

Extend the aggregation pipeline to use $divide to divide totalWithTax by 2. Output the product and the result as averageRevenue.

MongoDB

db.sales.insertMany([
  { product: "Pen", quantity: 10, price: 2 },
  { product: "Notebook", quantity: 5, price: 4 },
  { product: "Eraser", quantity: 20, price: 1 }
])

const tax = 5

db.sales.aggregate([
  {
    $project: {
      product: 1,
      totalWithTax: { $add: [ { $multiply: ["$quantity", "$price"] }, tax ] }
    }
  }
])

// Add $divide to calculate averageRevenue below
// Your code here

Hint

Use $divide inside $project to divide the sum by 2 and rename the field to averageRevenue.

Practice

1. What does the MongoDB expression { $add: [5, 3] } do?

easy

A. Multiplies 5 and 3 to get 15

B. Adds 5 and 3 to get 8

C. Divides 5 by 3 to get 1.67

D. Subtracts 3 from 5 to get 2

Solution

1. Step 1: Understand the $add operator

   The $add operator sums the numbers inside the array.

2. Step 2: Calculate the sum of 5 and 3

   5 + 3 equals 8.

3. Final Answer:

   Adds 5 and 3 to get 8 -> Option B

4. Quick Check:

   $add: [5, 3] = 8 [OK]

Hint: Remember $add sums all values inside the array [OK]

Common Mistakes:

• Confusing $add with $multiply
• Thinking $add subtracts values
• Assuming $add divides numbers

2. Which of the following is the correct syntax to multiply 4 by 7 using MongoDB arithmetic expressions?

easy

A. { $multiply: [4, 7] }

B. { $multiply: 4, 7 }

C. { $multiply: (4, 7) }

D. { multiply: [4, 7] }

Solution

1. Step 1: Recall correct operator syntax

   MongoDB arithmetic operators use $ followed by the operator name and an array of values.

2. Step 2: Check each option

   { $multiply: [4, 7] } uses $multiply with an array [4, 7], which is correct syntax.

3. Final Answer:

   { $multiply: [4, 7] } -> Option A

4. Quick Check:

   Correct syntax uses $multiply with array [OK]

Hint: Use square brackets [] for values in $multiply [OK]

Common Mistakes:

• Omitting $ before multiply
• Using parentheses instead of brackets
• Passing values without array brackets

3. Given the MongoDB aggregation expression { $divide: [ { $multiply: [10, 5] }, 2 ] }, what is the result?

medium

A. 25

B. 100

C. 2.5

D. 50

Solution

1. Step 1: Calculate the inner $multiply expression

   $multiply: [10, 5] equals 50.

2. Step 2: Divide the result by 2

   $divide: [50, 2] equals 25.

3. Final Answer:

   25 -> Option A

4. Quick Check:

   (10 * 5) / 2 = 25 [OK]

Hint: Calculate inner expressions first, then outer [OK]

Common Mistakes:

• Dividing 10 by 5 instead of multiplying
• Forgetting to divide after multiplication
• Mixing order of operations

4. Identify the error in this MongoDB expression: { $divide: [10, { $add: 2, 3 }] }

medium

A. There is no error; the expression is correct

B. The $divide operator cannot divide numbers

C. The numbers 10, 2, and 3 must be strings

D. The $add operator must have an array of values, not separate arguments

Solution

1. Step 1: Check $add operator usage

   $add requires an array of values, but { $add: 2, 3 } uses separate arguments incorrectly.

2. Step 2: Confirm correct syntax

   The correct syntax is { $add: [2, 3] } with square brackets.

3. Final Answer:

   The $add operator must have an array of values, not separate arguments -> Option D

4. Quick Check:

   $add needs array syntax [OK]

Hint: Always use square brackets [] for $add values [OK]

Common Mistakes:

• Using commas instead of array brackets
• Assuming $divide cannot divide numbers
• Passing numbers as strings

5. You want to calculate the average price of items stored in a MongoDB collection using aggregation. Which expression correctly calculates the average price if you have total price as { $add: ["$price1", "$price2", "$price3"] } and want to divide by 3?

hard

A. { $multiply: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] }

B. { $add: [ { $divide: ["$price1", 3] }, { $divide: ["$price2", 3] }, { $divide: ["$price3", 3] } ] }

C. { $divide: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] }

D. { $divide: [ 3, { $add: ["$price1", "$price2", "$price3"] } ] }

Solution

1. Step 1: Understand the average formula

   Average = (sum of values) / number of values.

2. Step 2: Check each option

   { $divide: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] } sums prices then divides by 3, matching the average formula.

3. Final Answer:

   { $divide: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] } -> Option C

4. Quick Check:

   Sum prices then divide by count = average [OK]

Hint: Sum first, then divide by count for average [OK]

Common Mistakes:

• Dividing each price separately then adding
• Multiplying sum by count instead of dividing
• Dividing count by sum instead of sum by count