$lt and $lte for less than in MongoDB - Mini Project: Build & Apply

$lt and $lte for less than in MongoDB

📖 Scenario: You are managing a small online bookstore database. You want to find books priced below certain amounts to offer discounts or promotions.

🎯 Goal: Build MongoDB queries using $lt and $lte operators to find books with prices less than or less than or equal to given values.

📋 What You'll Learn

Create a collection called books with sample book documents including title and price fields

Define a variable maxPrice to set the price limit

Write a query using $lt to find books priced less than maxPrice

Write a query using $lte to find books priced less than or equal to maxPrice

💡 Why This Matters

🌍 Real World

Filtering products or items in a database by price or other numeric values is common in e-commerce and inventory management.

💼 Career

Understanding MongoDB query operators like $lt and $lte is essential for backend developers and data analysts working with NoSQL databases.

1

Create the books collection with sample data

Create a MongoDB collection called books and insert these exact documents: { title: "Book A", price: 15 }, { title: "Book B", price: 20 }, { title: "Book C", price: 25 }

MongoDB

// Insert the sample book documents into the books collection
// Your code here

Hint

Use db.books.insertMany() with an array of book objects.

2

Define the price limit variable

Create a variable called maxPrice and set it to 20 to use as the price limit for queries.

MongoDB

db.books.insertMany([
  { title: "Book A", price: 15 },
  { title: "Book B", price: 20 },
  { title: "Book C", price: 25 }
])

// Define maxPrice variable
// Your code here

Hint

Use const maxPrice = 20 to create the variable.

3

Write a query using $lt to find books priced less than maxPrice

Write a MongoDB query using db.books.find() with the condition { price: { $lt: maxPrice } } to find books priced less than maxPrice.

MongoDB

db.books.insertMany([
  { title: "Book A", price: 15 },
  { title: "Book B", price: 20 },
  { title: "Book C", price: 25 }
])

const maxPrice = 20;

// Write the query to find books with price less than maxPrice
// Your code here

Hint

Use { price: { $lt: maxPrice } } inside find().

4

Write a query using $lte to find books priced less than or equal to maxPrice

Write a MongoDB query using db.books.find() with the condition { price: { $lte: maxPrice } } to find books priced less than or equal to maxPrice.

MongoDB

db.books.insertMany([
  { title: "Book A", price: 15 },
  { title: "Book B", price: 20 },
  { title: "Book C", price: 25 }
])

const maxPrice = 20;

db.books.find({ price: { $lt: maxPrice } })

// Write the query to find books with price less than or equal to maxPrice
// Your code here

Hint

Use { price: { $lte: maxPrice } } inside find().

Practice

1. What does the MongoDB operator $lt do in a query?

easy

A. Finds documents where the field value is less than a specified value

B. Finds documents where the field value is greater than a specified value

C. Finds documents where the field value is equal to a specified value

D. Finds documents where the field value is not equal to a specified value

Solution

1. Step 1: Understand the meaning of $lt

   The $lt operator means "less than" in MongoDB queries.

2. Step 2: Apply the operator meaning to query behavior

   It selects documents where the field value is strictly less than the given value.

3. Final Answer:

   Finds documents where the field value is less than a specified value -> Option A

4. Quick Check:

   $lt means less than [OK]

Hint: Remember: lt = less than, lte = less than or equal [OK]

Common Mistakes:

• Confusing $lt with $lte
• Thinking $lt means less than or equal
• Mixing $lt with greater than operators

2. Which of the following is the correct MongoDB query syntax to find documents where the field age is less than or equal to 30?

easy

A. { age: { $lte: 30 } }

B. { age: { $lt: 30 } }

C. { age: { $gt: 30 } }

D. { age: { $gte: 30 } }

Solution

1. Step 1: Identify the operator for less than or equal

   The operator $lte means "less than or equal to" in MongoDB.

2. Step 2: Match the operator with the query syntax

   The correct syntax to find documents where age is less than or equal to 30 is { age: { $lte: 30 } }.

3. Final Answer:

   { age: { $lte: 30 } } -> Option A

4. Quick Check:

   $lte means less than or equal [OK]

Hint: Use $lte for less than or equal, $lt for strictly less [OK]

Common Mistakes:

• Using $lt instead of $lte for less than or equal
• Using greater than operators by mistake
• Incorrect query object structure

3. Given the collection products with documents:

{ "name": "Pen", "price": 5 }

{ "name": "Notebook", "price": 10 }

{ "name": "Bag", "price": 20 }

What will be the result of the query db.products.find({ price: { $lt: 10 } })?

medium

A. []

B. [{ "name": "Pen", "price": 5 }, { "name": "Notebook", "price": 10 }]

C. [{ "name": "Pen", "price": 5 }]

D. [{ "name": "Notebook", "price": 10 }, { "name": "Bag", "price": 20 }]

Solution

1. Step 1: Understand the query condition

   The query uses $lt: 10, so it finds documents where price is less than 10.

2. Step 2: Check each document's price

   Only the "Pen" has price 5, which is less than 10. "Notebook" has price 10, which is not less than 10.

3. Final Answer:

   [{ "name": "Pen", "price": 5 }] -> Option C

4. Quick Check:

   Only prices less than 10 included [OK]

Hint: Remember $lt excludes equal values, only strictly less [OK]

Common Mistakes:

• Including documents with price equal to 10
• Confusing $lt with $lte
• Returning empty result by mistake

4. You wrote this query to find documents where score is less than or equal to 50:
db.scores.find({ score: { $lt: 50 } })
But you want to include documents where score is exactly 50. What is the error and how to fix it?

medium

A. Use $gt instead of $lt

B. No error, query is correct

C. Add $eq: 50 inside the query

D. Use $lte instead of $lt to include 50

Solution

1. Step 1: Identify the operator used

   The query uses $lt, which excludes values equal to 50.

2. Step 2: Choose the correct operator to include 50

   To include values equal to 50, use $lte (less than or equal).

3. Final Answer:

   Use $lte instead of $lt to include 50 -> Option D

4. Quick Check:

   $lte includes equal values [OK]

Hint: Use $lte to include the boundary value [OK]

Common Mistakes:

• Using $lt and expecting equal values included
• Using $gt which is wrong direction
• Trying to combine $lt and $eq unnecessarily

5. You have a collection orders with documents containing total values. You want to find all orders with total less than or equal to 100 but greater than 50. Which query correctly uses $lt and $lte to achieve this?

hard

A. { total: { $gte: 50, $lt: 100 } }

B. { total: { $gt: 50, $lte: 100 } }

C. { total: { $lt: 100, $lte: 50 } }

D. { total: { $lt: 100, $gt: 50 } }

Solution

1. Step 1: Understand the range needed

   We want totals greater than 50 and less than or equal to 100.

2. Step 2: Match operators to conditions

   $gt: 50 means strictly greater than 50, $lte: 100 means less than or equal to 100.

3. Step 3: Check query syntax

   { total: { $gt: 50, $lte: 100 } } correctly uses both operators in one object: { total: { $gt: 50, $lte: 100 } }.

4. Final Answer:

   { total: { $gt: 50, $lte: 100 } } -> Option B

5. Quick Check:

   Use $gt for greater than, $lte for less than or equal [OK]

Hint: Combine $gt and $lte for range queries [OK]

Common Mistakes:

• Mixing $lt and $lte incorrectly
• Using $gte instead of $gt when strict greater needed
• Reversing operator directions