Combining comparison operators in MongoDB - Mini Project: Build & Apply

Combining Comparison Operators in MongoDB Queries

📖 Scenario: You are managing a small online bookstore database. You want to find books that meet specific price and rating criteria to recommend to customers.

🎯 Goal: Build a MongoDB query that uses combined comparison operators to find books priced between $10 and $30 and with a rating greater than 4.

📋 What You'll Learn

Create a collection named books with sample book documents

Add a variable for the price range limits

Write a MongoDB query using $gte, $lte, and $gt operators combined with $and

Complete the query to find books matching the criteria

💡 Why This Matters

🌍 Real World

Filtering products or items in a database based on multiple conditions is common in online stores, inventory systems, and recommendation engines.

💼 Career

Understanding how to combine comparison operators in queries is essential for database querying roles, backend development, and data analysis.

1

Create the books collection with sample data

Create a variable called books and assign it a list of three book documents. Each document should have these exact fields and values: { title: "Book A", price: 15, rating: 4.5 }, { title: "Book B", price: 25, rating: 3.9 }, and { title: "Book C", price: 35, rating: 4.8 }.

MongoDB

# Create the books list with three book documents
# Your code here

Hint

Use a list with three dictionaries, each representing a book with the exact fields and values.

2

Define the price range limits

Create two variables called min_price and max_price and set them to 10 and 30 respectively.

MongoDB

books = [
    { "title": "Book A", "price": 15, "rating": 4.5 },
    { "title": "Book B", "price": 25, "rating": 3.9 },
    { "title": "Book C", "price": 35, "rating": 4.8 }
]
# Define min_price and max_price variables
# Your code here

Hint

Set min_price to 10 and max_price to 30 as numbers.

3

Write the MongoDB query with combined comparison operators

Create a variable called query and assign it a MongoDB query object that uses $and to combine these conditions: price greater than or equal to min_price, price less than or equal to max_price, and rating greater than 4. Use $gte, $lte, and $gt operators exactly as shown.

MongoDB

books = [
    { "title": "Book A", "price": 15, "rating": 4.5 },
    { "title": "Book B", "price": 25, "rating": 3.9 },
    { "title": "Book C", "price": 35, "rating": 4.8 }
]
min_price = 10
max_price = 30
# Create the query using $and with $gte, $lte, and $gt
# Your code here

Hint

Use $and with a list of conditions combining $gte, $lte, and $gt for the fields.

4

Complete the query to find matching books

Create a variable called matching_books and assign it the result of filtering the books list using the query conditions. Use a list comprehension that checks each book's price and rating against the query criteria exactly as defined.

MongoDB

books = [
    { "title": "Book A", "price": 15, "rating": 4.5 },
    { "title": "Book B", "price": 25, "rating": 3.9 },
    { "title": "Book C", "price": 35, "rating": 4.8 }
]
min_price = 10
max_price = 30
query = {
    "$and": [
        { "price": { "$gte": min_price } },
        { "price": { "$lte": max_price } },
        { "rating": { "$gt": 4 } }
    ]
}
# Filter books using the query conditions
# Your code here

Hint

Use a list comprehension to filter books matching all query conditions.

Practice

1. Which MongoDB query correctly finds documents where the field age is greater than 20 and less than 30?

easy

A. { age: { $gte: 20, $lte: 30 } }

B. { age: { $gt: 20, $lt: 30 } }

C. { age: { $ne: 20, $ne: 30 } }

D. { age: { $gt: 20 }, $lt: 30 }

Solution

1. Step 1: Understand the requirement for age range

   The question asks for documents where age is strictly greater than 20 and strictly less than 30.

2. Step 2: Identify the correct MongoDB syntax for combined conditions

   Using { age: { $gt: 20, $lt: 30 } } combines both conditions on the same field correctly.

3. Final Answer:

   { age: { $gt: 20, $lt: 30 } } -> Option B

4. Quick Check:

   Combined $gt and $lt on same field = { age: { $gt: 20, $lt: 30 } } [OK]

Hint: Use $gt and $lt together inside one field object [OK]

Common Mistakes:

• Using $gte and $lte includes boundary values, not strictly between
• Placing $lt outside the field object causes syntax error
• Using $ne does not check range but excludes specific values

2. Which of the following is the correct syntax to find documents where score is not equal to 50 and less than or equal to 100?

easy

A. { score: { $ne: 50, $lte: 100 } }

B. { score: { $ne: 50 }, $lte: 100 }

C. { score: { $ne: 50, $lt: 100 } }

D. { score: { $ne: 50, $gte: 100 } }

Solution

1. Step 1: Combine $ne and $lte inside the same field object

   Both conditions must apply to score, so they go together inside one object.

2. Step 2: Check each option's syntax

   { score: { $ne: 50, $lte: 100 } } correctly combines $ne: 50 and $lte: 100 inside score. { score: { $ne: 50 }, $lte: 100 } separates $lte outside, which is invalid syntax.

3. Final Answer:

   { score: { $ne: 50, $lte: 100 } } -> Option A

4. Quick Check:

   Combine $ne and $lte inside one field object = { score: { $ne: 50, $lte: 100 } } [OK]

Hint: Put all comparison operators inside one field object [OK]

Common Mistakes:

• Placing $lte outside the field object causes syntax error
• Using $gte instead of $lte changes the condition
• Separating operators into different objects breaks query

3. Given the collection products with documents like { price: 150, stock: 30 }, what will the query { price: { $gt: 100, $lt: 200 }, stock: { $gte: 20, $lte: 40 } } return?

medium

A. Documents where price is greater than 100 or stock is less than 40

B. Documents where price is exactly 100 and stock is exactly 20

C. Documents where price is between 100 and 200, and stock is between 20 and 40

D. Documents where price is less than 100 and stock is greater than 40

Solution

1. Step 1: Understand the combined conditions on price and stock

   The query uses $gt and $lt on price to find values strictly between 100 and 200, and $gte and $lte on stock to find values between 20 and 40 inclusive.

2. Step 2: Interpret the logical AND behavior of MongoDB queries

   Both conditions must be true for a document to match, so only documents with price in (100,200) and stock in [20,40] are returned.

3. Final Answer:

   Documents where price is between 100 and 200, and stock is between 20 and 40 -> Option C

4. Quick Check:

   Combined conditions on fields = Documents where price is between 100 and 200, and stock is between 20 and 40 [OK]

Hint: All conditions on different fields combine with AND logic [OK]

Common Mistakes:

• Thinking conditions combine with OR instead of AND
• Confusing inclusive ($gte/$lte) with exclusive ($gt/$lt)
• Assuming exact matches instead of ranges

4. Identify the error in this MongoDB query: { quantity: { $gt: 10 }, $lt: 50 }

medium

A. The query is correct and will work as expected

B. The $gt operator should be $gte

C. The field name 'quantity' is missing

D. The $lt operator is outside the field object and causes syntax error

Solution

1. Step 1: Analyze the placement of comparison operators

   The $lt operator is placed outside the quantity field object, which is invalid syntax in MongoDB queries.

2. Step 2: Correct syntax requires all operators on the same field inside one object

   Both $gt and $lt must be inside the quantity object like { quantity: { $gt: 10, $lt: 50 } }.

3. Final Answer:

   The $lt operator is outside the field object and causes syntax error -> Option D

4. Quick Check:

   All operators must be inside field object = The $lt operator is outside the field object and causes syntax error [OK]

Hint: Keep all comparison operators inside the same field object [OK]

Common Mistakes:

• Placing operators outside the field object
• Confusing $gt with $gte unnecessarily
• Assuming query works despite syntax error

5. You want to find documents where rating is greater than 3 but not equal to 5, and reviews are between 10 and 100 inclusive. Which query correctly combines these conditions?

hard

A. { rating: { $gt: 3, $ne: 5 }, reviews: { $gte: 10, $lte: 100 } }

B. { rating: { $gt: 3 }, rating: { $ne: 5 }, reviews: { $gte: 10, $lte: 100 } }

C. { rating: { $gt: 3, $ne: 5, $gte: 10 }, reviews: { $lte: 100 } }

D. { rating: { $gt: 3, $ne: 5 }, reviews: { $gt: 10, $lt: 100 } }

Solution

1. Step 1: Combine multiple comparison operators on the same field correctly

   Both $gt: 3 and $ne: 5 must be inside the same rating object to apply both conditions.

2. Step 2: Use inclusive range operators for reviews

   Since reviews should be between 10 and 100 inclusive, use $gte: 10 and $lte: 100 inside the reviews object.

3. Step 3: Check for syntax errors and logical correctness

   { rating: { $gt: 3, $ne: 5 }, reviews: { $gte: 10, $lte: 100 } } correctly combines all conditions. { rating: { $gt: 3 }, rating: { $ne: 5 }, reviews: { $gte: 10, $lte: 100 } } repeats the rating field, which is invalid. { rating: { $gt: 3, $ne: 5, $gte: 10 }, reviews: { $lte: 100 } } incorrectly adds $gte: 10 to rating. { rating: { $gt: 3, $ne: 5 }, reviews: { $gt: 10, $lt: 100 } } uses exclusive operators for reviews, which is incorrect.

4. Final Answer:

   { rating: { $gt: 3, $ne: 5 }, reviews: { $gte: 10, $lte: 100 } } -> Option A

5. Quick Check:

   Combine all conditions inside field objects correctly = { rating: { $gt: 3, $ne: 5 }, reviews: { $gte: 10, $lte: 100 } } [OK]

Hint: Put all conditions for one field inside one object, use correct operators [OK]

Common Mistakes:

• Repeating the same field multiple times in query
• Mixing inclusive and exclusive operators incorrectly
• Adding unrelated operators inside wrong field objects