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.