$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.

Practice

1. What does the $not operator do in MongoDB queries?

easy

A. It selects documents where the condition inside $not is false.

B. It selects documents where the condition inside $not is true.

C. It deletes documents that match the condition.

D. It updates documents that do not match the condition.

Solution

1. Step 1: Understand the purpose of $not

   The $not operator reverses the condition it wraps, so it matches documents where the condition is false.

2. Step 2: Apply this understanding to the options

   It selects documents where the condition inside $not is false. correctly states that $not selects documents where the condition inside it is false, which is the correct behavior.

3. Final Answer:

   It selects documents where the condition inside $not is false. -> Option A

4. Quick Check:

   $not flips condition = false [OK]

Hint: Remember: $not means 'not matching' condition [OK]

Common Mistakes:

• Thinking $not selects where condition is true
• Confusing $not with delete or update operations
• Using $not without a condition inside

2. Which of the following is the correct syntax to use $not with a comparison operator in MongoDB?

easy

A. { field: { $not: { $gt: 10 } } }

B. { field: { $gt: { $not: 10 } } }

C. { $not: { field: $gt: 10 } }

D. { field: { $not: $gt: 10 } }

Solution

1. Step 1: Review correct $not syntax

   The $not operator must wrap another operator inside the field, like { field: { $not: { $gt: 10 } } }.

2. Step 2: Check each option

   { field: { $not: { $gt: 10 } } } matches the correct syntax. Options A, B, and D misuse the placement or structure of $not.

3. Final Answer:

   { field: { $not: { $gt: 10 } } } -> Option A

4. Quick Check:

   Correct $not syntax wraps operator inside field [OK]

Hint: Wrap operator inside $not within the field object [OK]

Common Mistakes:

• Placing $not outside the field
• Not wrapping the operator inside $not
• Using invalid JSON structure with $not

3. Given the collection documents:

[{ "score": 5 }, { "score": 10 }, { "score": 15 }]

What will be the result of this query?

{ "score": { "$not": { "$gt": 10 } } }

medium

A. []

B. [{ "score": 15 }]

C. [{ "score": 5 }]

D. [{ "score": 5 }, { "score": 10 }]

Solution

1. Step 1: Understand the query condition

   The query uses $not with $gt: 10, so it matches documents where score is NOT greater than 10.

2. Step 2: Check each document against the condition

   Documents with scores 5 and 10 are not greater than 10, so they match. The document with 15 does not match.

3. Final Answer:

   [{ "score": 5 }, { "score": 10 }] -> Option D

4. Quick Check:

   Scores ≤ 10 match $not $gt 10 [OK]

Hint: Think: $not $gt 10 means ≤ 10 [OK]

Common Mistakes:

• Selecting documents where score > 10 instead
• Confusing $not with $ne
• Ignoring the nested operator inside $not

4. You wrote this query but it returns an error:

{ "name": { "$not": "^A" } }

What is the problem?

medium

A. The query is missing a closing brace.

B. $not must be used with another operator like $regex.

C. $not cannot be used on string fields.

D. The regex pattern is invalid without slashes.

Solution

1. Step 1: Analyze the use of $not with a string

   The $not operator expects an operator expression, not a direct string.

2. Step 2: Correct usage with regex

   To negate a regex, you must use { "$not": { "$regex": "^A" } }. The given query misses $regex.

3. Final Answer:

   $not must be used with another operator like $regex. -> Option B

4. Quick Check:

   $not needs operator, not raw value [OK]

Hint: Always pair $not with an operator like $regex [OK]

Common Mistakes:

• Using raw string inside $not without operator
• Assuming regex needs slashes in MongoDB
• Thinking $not works on any value directly

5. You want to find documents where the status field does NOT start with the letter 'P'. Which query correctly uses $not with a regex to achieve this?

hard

A. { "status": { "$regex": { "$not": "^P" } } }

B. { "$not": { "status": $regex: "^P" } }

C. { "status": { "$not": { "$regex": "^P" } } }

D. { "status": { "$not": "^P" } }

Solution

1. Step 1: Understand the goal

   You want documents where status does NOT start with 'P', so negate the regex ^P.

2. Step 2: Use $not with $regex inside the field

   The correct syntax is { "status": { "$not": { "$regex": "^P" } } }. This matches documents where status does not match the regex.

3. Final Answer:

   { "status": { "$not": { "$regex": "^P" } } } -> Option C

4. Quick Check:

   $not wraps $regex inside field [OK]

Hint: Wrap regex inside $not within the field object [OK]

Common Mistakes:

• Placing $not outside the field
• Using $not directly on string without $regex
• Incorrect nesting of $not and $regex