Boolean and null types in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents where a field is explicitly false

Given a collection users with documents containing a subscribed field that can be true, false, or null, which query returns only users who have subscribed set to false (not null or missing)?

MongoDB

db.users.find({ subscribed: false })

A db.users.find({ subscribed: { $ne: true } })

B db.users.find({ subscribed: false })

C db.users.find({ subscribed: { $exists: false } })

D db.users.find({ subscribed: null })

💡 Hint

Remember that false is different from null and missing fields.

Query B matches documents where subscribed is exactly false (not null or missing). Query A matches documents where subscribed exists and is not true, so it includes false and null. Query C matches documents where subscribed does not exist. Query D matches documents where subscribed is null.

🧠 Conceptual

intermediate

Understanding null vs missing fields

In MongoDB, what is the difference between a field set to null and a field that is missing in a document?

A A field set to <code>null</code> exists with a null value; a missing field does not exist in the document.

B There is no difference; both are treated the same in queries.

C A field set to <code>null</code> is ignored in queries, but missing fields cause errors.

D A missing field is stored as <code>null</code> internally by MongoDB.

💡 Hint

Think about whether the field is present in the document or not.

A field set to null is present in the document with a value of null. A missing field is not present at all. This affects how queries like {field: null} behave.

📝 Syntax

advanced

Correctly query documents with boolean true or null

Which MongoDB query correctly finds documents where the active field is either true or null?

MongoDB

db.collection.find({ $or: [{ active: true }, { active: null }] })

A db.collection.find({ active: { $in: [true, null] } })

B db.collection.find({ active: true || null })

C db.collection.find({ active: { $or: [true, null] } })

D db.collection.find({ $or: [{ active: true }, { active: null }] })

💡 Hint

Remember how to combine conditions with $or in MongoDB.

Option D uses $or correctly to find documents where active is true or null. Option D uses $in which also works but is less explicit. Option D is invalid syntax. Option D uses $or incorrectly inside a field.

❓ optimization

advanced

Efficiently query documents with a boolean field false or missing

You want to find documents where the verified field is either false or does not exist. Which query is the most efficient and correct?

A db.users.find({ $or: [{ verified: false }, { verified: { $exists: false } }] })

B db.users.find({ verified: { $ne: true } })

C db.users.find({ verified: false, verified: { $exists: false } })

D db.users.find({ verified: { $in: [false, null] } })

💡 Hint

Think about how to combine conditions for false and missing fields.

Option A explicitly checks for verified being false or missing using $or and $exists. Option A matches documents where verified exists and is not true, so includes false and null (but not missing fields or other values). Option A is invalid because it tries to match two conditions on the same field in one object. Option A includes false and null (existing fields) but not missing fields.

🔧 Debug

expert

Identify the error in a query filtering null and false

What error or issue occurs with this MongoDB query?

db.orders.find({ status: false || null })

A The query matches documents where status is false only, ignoring null.

B The query returns all documents because false || null evaluates to false.

C The query matches documents where status is null only, ignoring false.

D SyntaxError because MongoDB does not allow logical operators inside query values.

💡 Hint

Consider how JavaScript evaluates expressions before sending to MongoDB.

The expression false || null evaluates to null in JavaScript before the query runs. So the query becomes { status: null }, ignoring false. This is not a syntax error but a logic mistake.