$nor operator behavior in MongoDB

Introduction

The $nor operator helps find documents that do not match any of the given conditions. It is like saying "none of these" in a search.

You want to find users who are neither from New York nor from California.

You want to get products that are not cheap and not on sale.

You want to exclude documents that have either a missing field or a specific value.

You want to filter out records that match any of several unwanted conditions.

Syntax

MongoDB

{ $nor: [ { condition1 }, { condition2 }, ... ] }

The $nor operator takes an array of conditions.

It returns documents where none of the conditions are true.

Examples

Finds documents where age is neither less than 18 nor greater than 65.

MongoDB

{ $nor: [ { age: { $lt: 18 } }, { age: { $gt: 65 } } ] }

Finds documents where status is not "A" and qty is not less than 30.

MongoDB

{ $nor: [ { status: "A" }, { qty: { $lt: 30 } } ] }

Sample Program

This query finds documents where status is not "A" and qty is not less than 50. It excludes any document matching either condition.

MongoDB

db.inventory.insertMany([
  { item: "journal", qty: 25, status: "A" },
  { item: "notebook", qty: 50, status: "A" },
  { item: "paper", qty: 100, status: "D" },
  { item: "planner", qty: 75, status: "D" },
  { item: "postcard", qty: 45, status: "A" }
])

// Find items where status is neither "A" nor qty less than 50

const result = db.inventory.find({ $nor: [ { status: "A" }, { qty: { $lt: 50 } } ] }).toArray()

printjson(result)

Important Notes

The $nor operator is the opposite of $or. It returns documents that do not satisfy any of the conditions.

If the array is empty, $nor matches all documents.

Summary

$nor finds documents where none of the given conditions are true.

It takes an array of conditions and excludes documents matching any of them.

Useful to filter out unwanted records based on multiple criteria.

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