Implicit AND with multiple conditions in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents matching multiple conditions with implicit AND

Given a collection products with documents containing category and price, what is the output of this query?

db.products.find({ category: "books", price: { $lt: 20 } })

Assume the collection has:

{"category": "books", "price": 15}

{"category": "books", "price": 25}

{"category": "electronics", "price": 15}

MongoDB

db.products.find({ category: "books", price: { $lt: 20 } })

A [{"category": "books", "price": 25}]

B [{"category": "books", "price": 15}]

C [{"category": "electronics", "price": 15}]

D []

💡 Hint

Remember that specifying multiple fields in the query object means all conditions must be true.

The query matches documents where category is 'books' AND price is less than 20. Only the first document meets both conditions.

🧠 Conceptual

intermediate

Understanding implicit AND in MongoDB queries

Which statement best describes how MongoDB treats multiple conditions inside a single query object?

A MongoDB treats multiple conditions as an implicit OR, returning documents matching any condition.

B MongoDB ignores all but the first condition in the query object.

C MongoDB requires explicit $and operator to combine multiple conditions.

D MongoDB treats multiple conditions as an implicit AND, returning documents matching all conditions.

💡 Hint

Think about how multiple fields in a JSON object combine logically.

MongoDB combines multiple fields in a query object with an implicit AND, so all conditions must be true for a document to match.

📝 Syntax

advanced

Identify the invalid MongoDB query with multiple conditions

Which of the following MongoDB queries will cause a syntax error or fail to run?

A db.collection.find({ age: { $gt: 18 }, status: { $in: ["active", "pending"] } })

B db.collection.find({ $and: [{ age: { $gt: 18 } }, { status: "active" }] })

C db.collection.find({ age: { $gt: 18 }, $or: { status: "active" } })

D db.collection.find({ age: { $gt: 18 }, status: "active" })

💡 Hint

Check if the query mixes implicit AND with explicit $or incorrectly.

Option C is invalid because it mixes implicit AND with an $or operator at the same level without proper structure, causing a syntax error.

❓ optimization

advanced

Optimizing queries with multiple conditions

You want to find documents where status is "active" and score is greater than 50. Which query is more efficient in MongoDB?

A db.collection.find({ status: "active", score: { $gt: 50 } })

B db.collection.find({ $and: [{ status: "active" }, { score: { $gt: 50 } }] })

C db.collection.find({ $or: [{ status: "active" }, { score: { $gt: 50 } }] })

D db.collection.find({ status: { $eq: "active" }, score: { $gt: 50 } })

💡 Hint

Consider how MongoDB processes implicit AND vs explicit $and.

Option A uses implicit AND which MongoDB optimizes internally and is simpler and more efficient than explicit $and.

🔧 Debug

expert

Why does this MongoDB query return no results?

Given the collection users with documents:

{"name": "Alice", "age": 30, "city": "NY"}

{"name": "Bob", "age": 35, "city": "LA"}

Why does this query return no documents?

db.users.find({ age: { $gt: 20 }, age: { $lt: 28 } })

A Because MongoDB does not support multiple conditions on the same field in one query object; the second overwrites the first.

B Because no user has age greater than 20 and less than 28 at the same time.

C Because the query syntax is invalid and causes an error.

D Because the collection is empty.

💡 Hint

Check how JSON objects handle duplicate keys.

In JSON, duplicate keys are overwritten. The second 'age' condition replaces the first, so only { age: { $lt: 28 } } is applied, which matches users with age less than 28. The query returns no results because no user matches that condition. But the user expects both conditions combined.