Combining comparison operators in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with age between 25 and 35 inclusive

Given a collection users with documents containing an age field, which query returns all users whose age is between 25 and 35 inclusive?

MongoDB

db.users.find({ age: { $gte: 25, $lte: 35 } })

A db.users.find({ age: { $gte: 25 }, age: { $lte: 35 } })

B db.users.find({ age: { $gt: 25, $lt: 35 } })

C db.users.find({ age: { $gte: 25, $lte: 35 } })

D db.users.find({ age: { $gte: 25 } }, { age: { $lte: 35 } })

💡 Hint

Use $gte and $lte together inside the same object for the field.

Option C correctly combines $gte and $lte inside the same object for the age field, returning ages from 25 to 35 inclusive. Option C excludes 25 and 35. Option C is invalid because the same field cannot appear twice in the same object. Option C uses projection syntax incorrectly.

❓ query_result

intermediate

Find documents where score is less than 50 or greater than 90

Which MongoDB query returns documents where the score field is either less than 50 or greater than 90?

MongoDB

db.scores.find({ $or: [ { score: { $lt: 50 } }, { score: { $gt: 90 } } ] })

A db.scores.find({ score: { $lt: 50, $gt: 90 } })

B db.scores.find({ score: { $lt: 50 } }, { score: { $gt: 90 } })

C db.scores.find({ $and: [ { score: { $lt: 50 } }, { score: { $gt: 90 } } ] })

D db.scores.find({ $or: [ { score: { $lt: 50 } }, { score: { $gt: 90 } } ] })

💡 Hint

Use $or to combine conditions that can be true alternatively.

Option D correctly uses $or to find documents where score is less than 50 or greater than 90. Option D tries to combine $lt and $gt on the same field in one object, which is impossible to satisfy. Option D uses projection syntax incorrectly. Option D uses $and which requires both conditions to be true simultaneously, which is impossible.

📝 Syntax

advanced

Identify the syntax error in this query combining comparison operators

Which option contains a syntax error when trying to find documents with price between 100 and 200?

MongoDB

db.products.find({ price: { $gte: 100, $lte: 200 } })

A db.products.find({ price: { $gte: 100, $lte: 200 } })

B db.products.find({ price: { $gte: 100 }, price: { $lte: 200 } })

C db.products.find({ price: { $gte: 100, $lte: 200 } }, {})

D db.products.find({ price: { $gte: 100, $lte: 200 }, })

💡 Hint

Check if the same field appears twice in the same object.

Option B is invalid syntax because the field 'price' appears twice in the same object, which is not allowed in MongoDB queries. Options A, B, and C are syntactically valid. Option B includes an empty projection which is allowed. Option B has a trailing comma which is allowed in JavaScript objects.

❓ optimization

advanced

Optimize query to find documents with age not between 20 and 30

Which query is the most efficient and correct way to find documents where age is NOT between 20 and 30 inclusive?

A db.users.find({ $or: [ { age: { $lt: 20 } }, { age: { $gt: 30 } } ] })

B db.users.find({ age: { $not: { $gte: 20, $lte: 30 } } })

C db.users.find({ age: { $ne: { $gte: 20, $lte: 30 } } })

D db.users.find({ age: { $lt: 20, $gt: 30 } })

💡 Hint

Use $or to combine conditions that exclude the range.

Option A correctly uses $or to find ages less than 20 or greater than 30. Option A is invalid because $not cannot combine multiple operators inside one object like that. Option A is invalid syntax; $ne expects a value, not a range. Option A is impossible because age cannot be less than 20 and greater than 30 simultaneously.

🧠 Conceptual

expert

Understanding behavior of combined comparison operators in MongoDB

Consider the query db.collection.find({ value: { $gt: 10, $lt: 20 } }). Which statement about this query is true?

A It returns documents where value is greater than 10 and less than 20.

B It returns documents where value is greater than 10 or less than 20.

C It returns documents where value is exactly 10 or 20.

D It causes a syntax error because multiple operators cannot be combined.

💡 Hint

Think about how MongoDB interprets multiple operators inside one field object.

Option A is correct because MongoDB treats multiple comparison operators inside the same field object as an AND condition. Option A is incorrect because $or is needed for OR logic. Option A is incorrect because $gt and $lt exclude the boundary values. Option A is incorrect because combining multiple operators inside one object is valid syntax.

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