$nor operator behavior in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $nor operator do in MongoDB?

The $nor operator performs a logical NOR operation on an array of query expressions. It returns documents that fail all the given conditions, meaning none of the conditions are true.

intermediate

How is $nor different from $not in MongoDB?

$nor takes an array of conditions and returns documents where none of the conditions are true. $not negates a single condition. $nor is like combining multiple $not conditions with AND logic.

beginner

Example: What documents does this query return?
{ $nor: [ { age: { $lt: 30 } }, { status: "A" } ] }

This query returns documents where age is NOT less than 30 AND status is NOT "A". So, only documents that fail both conditions are returned.

intermediate

Can $nor be used with a single condition array? What happens?

Yes, $nor can be used with a single condition inside an array. It returns documents where that single condition is false, effectively negating that condition.

advanced

What happens if the $nor array is empty in a query?

If the $nor array is empty, the query matches all documents because there are no conditions to fail, so none of the conditions are true by default.

What does the $nor operator return in MongoDB?

A Documents where none of the specified conditions are true

B Documents where all specified conditions are true

C Documents where at least one condition is true

D Documents where exactly one condition is true

$nor returns documents that fail all the given conditions.

Which of these is equivalent to { $nor: [ { age: { $lt: 30 } } ] }?

A { age: { $lte: 30 } }

B { age: { $gte: 30 } }

C { age: { $gt: 30 } }

D { age: { $lt: 30 } }

$nor with one condition negates it, so it matches documents where age is greater than or equal to 30.

If $nor is given an empty array, what documents are returned?

A No documents

B Only documents with null fields

C Documents with at least one true condition

D All documents

An empty $nor means no conditions to fail, so all documents match.

How does $nor combine multiple conditions?

A Returns documents where all conditions are true

B Returns documents where any condition is true

C Returns documents where all conditions are false

D Returns documents where exactly one condition is true

$nor returns documents where none of the conditions are true.

Which logical operator is $nor most similar to?

A NOT OR

B AND

C OR

D XOR

$nor means NOT (condition1 OR condition2 ...), so it is a NOT OR.

Explain how the $nor operator works in MongoDB queries.

Think about how it combines multiple conditions with NOT and OR.

Describe a real-life example where you might use $nor in a MongoDB query.

Imagine you want to exclude people younger than 30 or with status 'A'.

Practice

1. What does the $nor operator do in MongoDB queries?

easy

A. Finds documents where all specified conditions are true

B. Finds documents where at least one condition is true

C. Finds documents where none of the specified conditions are true

D. Finds documents that match exactly one condition

Solution

1. Step 1: Understand the purpose of $nor

   The $nor operator returns documents that do not satisfy any of the given conditions.

2. Step 2: Compare with other logical operators

   Unlike $and or $or, $nor excludes documents matching any condition in its array.

3. Final Answer:

   Finds documents where none of the specified conditions are true -> Option C

4. Quick Check:

   $nor excludes all matching conditions = B [OK]

Hint: Think: no conditions should be true for $nor [OK]

Common Mistakes:

• Confusing $nor with $or
• Assuming it returns documents matching any condition
• Thinking it requires all conditions to be true

2. Which of the following is the correct syntax to use $nor in a MongoDB query to exclude documents where age is 25 or status is "active"?

easy

A. { $nor: { $or: [ { age: 25 }, { status: "active" } ] } }

B. { $nor: { age: 25, status: "active" } }

C. { $nor: [ age: 25, status: "active" ] }

D. { $nor: [ { age: 25 }, { status: "active" } ] }

Solution

1. Step 1: Recall $nor syntax

   $nor requires an array of condition objects inside square brackets.

2. Step 2: Check each option's structure

   { $nor: [ { age: 25 }, { status: "active" } ] } correctly uses an array of conditions. Options B and D use objects incorrectly, and C has invalid array syntax.

3. Final Answer:

   { $nor: [ { age: 25 }, { status: "active" } ] } -> Option D

4. Quick Check:

   Array of conditions inside $nor = A [OK]

Hint: Use square brackets for conditions array in $nor [OK]

Common Mistakes:

• Using curly braces instead of square brackets for conditions
• Passing a single object instead of an array
• Nesting $or inside $nor unnecessarily

3. Given the collection documents:

[{ "name": "Alice", "age": 30, "status": "active" }, { "name": "Bob", "age": 25, "status": "inactive" }, { "name": "Carol", "age": 35, "status": "active" }]

What will be the result of this query?

{ $nor: [ { age: 25 }, { status: "active" } ] }

medium

A. [] (empty array)

B. [{ "name": "Bob", "age": 25, "status": "inactive" }]

C. [{ "name": "Bob", "age": 25, "status": "inactive" }, { "name": "Carol", "age": 35, "status": "active" }]

D. [{ "name": "Bob", "age": 25, "status": "inactive" }, { "name": "Alice", "age": 30, "status": "active" }]

Solution

1. Step 1: Understand the $nor conditions

   The query excludes documents where age is 25 OR status is "active".

2. Step 2: Check each document against conditions

   Alice: status "active" -> excluded; Bob: age 25 -> excluded; Carol: status "active" -> excluded.

3. Final Answer:

   [] (empty array) -> Option A

4. Quick Check:

   All documents match at least one condition, so none returned = A [OK]

Hint: Exclude any document matching any condition in $nor [OK]

Common Mistakes:

• Returning documents that match one condition
• Confusing $nor with $or
• Assuming some documents pass when all match conditions

4. You wrote this MongoDB query but it throws an error:

{ $nor: { age: { $gt: 30 }, status: "inactive" } }

What is the problem and how to fix it?

medium

A. The field names must be strings; fix: { $nor: [ { "age": { $gt: 30 } }, { "status": "inactive" } ] }

B. The conditions must be inside an array; fix: { $nor: [ { age: { $gt: 30 } }, { status: "inactive" } ] }

C. The operator $gt is invalid; fix: use $gte instead

D. The query must use $and instead of $nor

Solution

1. Step 1: Identify the syntax error

   $nor expects an array of condition objects, but here it has a single object.

2. Step 2: Correct the syntax

   Wrap each condition inside its own object within an array to fix the error.

3. Final Answer:

   The conditions must be inside an array; fix: { $nor: [ { age: { $gt: 30 } }, { status: "inactive" } ] } -> Option B

4. Quick Check:

   $nor needs array of conditions = C [OK]

Hint: Always use an array of conditions with $nor [OK]

Common Mistakes:

• Passing a single object instead of an array
• Misusing comparison operators
• Confusing $nor with $and

5. You have a collection with documents:

[{ "item": "pen", "qty": 10, "status": "A" }, { "item": "pencil", "qty": 5, "status": "D" }, { "item": "notebook", "qty": 15, "status": "A" }, { "item": "eraser", "qty": 0, "status": "D" }]

Write a $nor query to find documents where qty is not 0 and status is not "D". Which query returns the correct documents?

hard

A. { $nor: [ { qty: 0 }, { status: "D" } ] }

B. { $nor: [ { qty: { $ne: 0 } }, { status: { $ne: "D" } } ] }

C. { $nor: [ { qty: { $eq: 0 } }, { status: { $ne: "D" } } ] }

D. { $nor: [ { qty: { $gt: 0 } }, { status: { $ne: "D" } } ] }

Solution

1. Step 1: Understand the requirement

   We want documents where qty is NOT 0 and status is NOT "D".

2. Step 2: Use $nor to exclude documents with qty 0 or status "D"

   { $nor: [ { qty: 0 }, { status: "D" } ] } excludes documents with qty 0 or status "D", so it returns documents matching the requirement.

3. Final Answer:

   { $nor: [ { qty: 0 }, { status: "D" } ] } -> Option A

4. Quick Check:

   Exclude unwanted values with $nor = D [OK]

Hint: Use $nor to exclude unwanted values directly [OK]

Common Mistakes:

• Using $ne inside $nor incorrectly
• Confusing inclusion with exclusion logic
• Using wrong comparison operators inside conditions