Combining logical and comparison operators in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents matching combined conditions

Given a collection products with fields price and category, which query returns all products with price greater than 100 and category either 'electronics' or 'appliances'?

MongoDB

db.products.find({ $and: [ { price: { $gt: 100 } }, { category: { $in: ['electronics', 'appliances'] } } ] })

A db.products.find({ price: { $gte: 100 }, category: { $nin: ['electronics', 'appliances'] } })

B db.products.find({ $or: [ { price: { $gt: 100 } }, { category: { $in: ['electronics', 'appliances'] } } ] })

C db.products.find({ $and: [ { price: { $lt: 100 } }, { category: { $in: ['electronics', 'appliances'] } } ] })

D db.products.find({ price: { $gt: 100 }, category: { $in: ['electronics', 'appliances'] } })

💡 Hint

Use $and to combine conditions that must both be true.

Option D uses implicit $and by specifying both conditions in the same object, which matches documents where price is greater than 100 and category is in the given list.

Option D uses $or, which matches documents where either condition is true, so it returns more documents.

Option D uses $lt (less than) for price, which is opposite of the requirement.

Option D uses $gte and $nin, which does not match the required conditions.

📝 Syntax

intermediate

Identify the syntax error in this MongoDB query

Which option contains a syntax error in combining logical and comparison operators to find users aged between 18 and 30 inclusive?

MongoDB

db.users.find({ age: { $gte: 18, $lte: 30 } })

A db.users.find({ age: { $gte: 18, $lte: 30 } })

B db.users.find({ $and: [ { age: { $gte: 18 } }, { age: { $lte: 30 } } ] })

C db.users.find({ age: { $gte: 18 }, age: { $lte: 30 } })

D db.users.find({ age: { $gte: 18, $lt: 30 } })

💡 Hint

Check if the object keys are repeated in the same level.

Option C repeats the age key twice in the same object, which is invalid syntax in MongoDB queries.

Option C correctly combines $gte and $lte in one object.

Option C uses $and with two separate conditions, which is valid.

Option C uses $gte and $lt correctly in one object.

❓ optimization

advanced

Optimize query combining multiple logical operators

Which query is the most efficient to find documents where status is 'active' and either score is above 80 or rank is below 10?

A db.records.find({ $and: [ { status: 'active' }, { $or: [ { score: { $gt: 80 } }, { rank: { $lt: 10 } } ] } ] })

B db.records.find({ status: 'active', $or: [ { score: { $gt: 80 } }, { rank: { $lt: 10 } } ] })

C db.records.find({ $or: [ { status: 'active', score: { $gt: 80 } }, { status: 'active', rank: { $lt: 10 } } ] })

D db.records.find({ $or: [ { status: 'active' }, { score: { $gt: 80 } }, { rank: { $lt: 10 } } ] })

💡 Hint

Use implicit $and by combining conditions in one object when possible.

Option B uses implicit $and by specifying status and $or conditions together, which is efficient and clear.

Option B uses explicit $and with $or, which works but is more verbose.

Option B repeats status inside each $or condition, which is redundant and less efficient.

Option B uses $or with status alone, which returns documents with status 'active' or score > 80 or rank < 10, not matching the requirement.

🔧 Debug

advanced

Debug why this query returns no results

A query is intended to find documents where age is between 20 and 40 and verified is true, but it returns no results. Which option explains the likely cause?

MongoDB

db.users.find({ $and: [ { age: { $gt: 20, $lt: 40 } }, { verified: true } ] })

A The query should use $gte and $lte to include ages 20 and 40; using $gt and $lt excludes boundary ages.

B The $and operator is invalid syntax in MongoDB queries, causing no results.

C The query uses $gt and $lt, so it excludes age 20 and 40; if no documents have age strictly between 20 and 40, no results appear.

D The verified field should be a string 'true' not boolean true, so no matches occur.

💡 Hint

Check if the comparison operators exclude boundary values.

Option A correctly identifies that $gt and $lt exclude ages exactly 20 and 40, so if documents have those ages, they won't match.

Option A is partially correct but less precise than C.

Option A is false; $and is valid syntax.

Option A assumes verified is string, but usually boolean true is correct.

🧠 Conceptual

expert

Understanding precedence of logical operators in MongoDB queries

Consider the query: db.collection.find({ $or: [ { status: 'A' }, { $and: [ { qty: { $lt: 30 } }, { price: { $gt: 20 } } ] } ] }). Which statement best describes how MongoDB evaluates this query?

A MongoDB returns documents where status is 'A' or both qty is less than 30 and price is greater than 20.

B MongoDB returns documents where status is 'A' and either qty is less than 30 or price is greater than 20.

C MongoDB returns documents where status is 'A' and qty is less than 30 and price is greater than 20.

D MongoDB returns documents where status is 'A' or qty is less than 30 or price is greater than 20.

💡 Hint

Recall that $and has higher precedence inside $or arrays.

The query uses $or with two conditions: one is status 'A', the other is an $and condition combining qty and price.

MongoDB returns documents matching either condition: status 'A' OR both qty < 30 AND price > 20.

Option A correctly describes this logic.

Options A, C, and D misunderstand the grouping and operator precedence.