$not operator behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of $not with $gt operator

Given a collection products with documents:

{ "name": "A", "price": 50 }

{ "name": "B", "price": 100 }

{ "name": "C", "price": 150 }

What documents are returned by this query?

{ price: { $not: { $gt: 100 } } }

A [{ "name": "A", "price": 50 }, { "name": "B", "price": 100 }]

B [{ "name": "C", "price": 150 }]

C [{ "name": "A", "price": 50 }]

D [{ "name": "B", "price": 100 }, { "name": "C", "price": 150 }]

💡 Hint

Remember $not inverts the condition inside it.

The query { price: { $not: { $gt: 100 } } } selects documents where price is NOT greater than 100. So prices 100 and below match.

🧠 Conceptual

intermediate

Understanding $not with regex

What does the following MongoDB query return?

{ name: { $not: /abc/ } }

Assuming documents have a name field with string values.

A Documents where <code>name</code> contains the substring 'abc'.

B Documents where <code>name</code> is missing or null.

C Documents where <code>name</code> is exactly 'abc'.

D Documents where <code>name</code> does NOT contain the substring 'abc'.

💡 Hint

Think about what $not does to the regex match.

The $not operator negates the regex match. So it returns documents where name does NOT match the regex /abc/.

📝 Syntax

advanced

Identify syntax error with $not usage

Which of the following MongoDB queries will cause a syntax error?

A { age: { $not: { $exists: true } } }

B { age: { $not: { $gte: 18 } } }

C { age: { $not: 18 } }

D { age: { $not: { $in: [18, 21] } } }

💡 Hint

Check the allowed value types for $not operator.

The $not operator requires a query expression or regex inside it. A direct value like 18 is invalid syntax.

❓ optimization

advanced

Optimizing $not queries

Which query is more efficient to find documents where score is NOT greater than 50?

A { score: { $lte: 50 } }

B { score: { $not: { $gt: 50 } } }

C { $nor: [ { score: { $gt: 50 } } ] }

D { score: { $lt: 51 } }

💡 Hint

Think about how indexes and query operators work.

Using $lte directly is more efficient because it can use indexes better than $not or $nor which invert conditions.

🔧 Debug

expert

Debugging unexpected $not behavior

A developer runs this query:

{ status: { $not: { $eq: "active" } } }

But it returns documents where status is missing or null, which was not expected.
Why does this happen?

A Because $eq inside $not is invalid and causes unexpected results.

B Because $not matches documents where the field does not exist or is null in addition to not matching the condition.

C Because $not only works with regex, not with $eq operator.

D Because the query syntax is incorrect and MongoDB ignores the $not operator.

💡 Hint

Consider how $not treats documents missing the field.

The $not operator matches documents where the condition inside is false or the field is missing or null. So documents without status also match.