$nin for not in set in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $nin operator do in MongoDB?

The $nin operator selects documents where the value of a field is not in the specified array of values.

beginner

How would you find documents where the field color is not 'red', 'blue', or 'green' using $nin?

{ color: { $nin: ['red', 'blue', 'green'] } }

This query returns documents where color is any value except 'red', 'blue', or 'green'.

intermediate

Can $nin be used with fields that contain arrays?

Yes, $nin checks if none of the values in the field's array are in the specified array. If any value matches, the document is excluded.

intermediate

What happens if you use $nin with an empty array like { field: { $nin: [] } }?

The query matches all documents because no values are excluded.

beginner

Is $nin the opposite of $in?

Yes, $nin selects documents where the field's value is not in the given array, while $in selects documents where the field's value is in the array.

Which MongoDB operator selects documents where a field's value is NOT in a list of values?

A $nin

B $in

C $eq

D $ne

$nin selects documents where the field's value is not in the specified array.

What will the query { age: { $nin: [20, 30, 40] } } return?

A Documents where age is 20, 30, or 40

B Documents where age is not 20, 30, or 40

C Documents where age is exactly 20

D Documents where age is missing

The query returns documents where age is not any of the values 20, 30, or 40.

If a field contains an array, how does $nin behave?

A Matches if any element is in the array

B Matches if all elements are in the array

C Matches if none of the elements are in the array

D Does not work with arrays

$nin matches documents where none of the array elements are in the specified list.

What does { field: { $nin: [] } } match?

A No documents

B All documents

C Documents where field is null

D Documents where field is an empty array

An empty array means no values are excluded, so all documents match.

Which operator is the direct opposite of $nin?

A $ne

B $in

C $eq

D $exists

$in selects documents where the field's value is in the array, opposite of $nin.

Explain how the $nin operator works in MongoDB queries.

Think about how you exclude certain items from a list.

Describe a real-life example where you might use $nin in a database query.

Imagine you want to find all fruits except apples and bananas.

Practice

1. What does the MongoDB operator $nin do in a query?

easy

A. Selects documents where the field's value is in the specified array

B. Selects documents where the field's value is NOT in the specified array

C. Updates documents with values in the specified array

D. Deletes documents where the field's value is in the specified array

Solution

1. Step 1: Understand the purpose of $nin

   The $nin operator is used to filter documents where a field's value is NOT included in a given list of values.

2. Step 2: Compare with other operators

   Unlike $in which selects values inside the array, $nin excludes those values.

3. Final Answer:

   Selects documents where the field's value is NOT in the specified array -> Option B

4. Quick Check:

   $nin excludes values = B [OK]

Hint: Remember: $nin means NOT in list [OK]

Common Mistakes:

• Confusing $nin with $in
• Thinking it updates or deletes documents
• Using it to select values inside the array

2. Which of the following is the correct syntax to find documents where the field status is NOT 'active' or 'pending' using $nin?

easy

A. { status: { $nin: ['active', 'pending'] } }

B. { status: { $nin: 'active', 'pending' } }

C. { status: { $nin: ['active'] || ['pending'] } }

D. { status: { $nin: 'active' && 'pending' } }

Solution

1. Step 1: Check the correct $nin syntax

   The $nin operator requires an array of values inside square brackets to specify the excluded set.

2. Step 2: Validate each option

   { status: { $nin: ['active', 'pending'] } } correctly uses an array with two strings. Options B, C, and D have syntax errors or invalid expressions.

3. Final Answer:

   { status: { $nin: ['active', 'pending'] } } -> Option A

4. Quick Check:

   Correct array syntax for $nin = A [OK]

Hint: Use square brackets for arrays in $nin [OK]

Common Mistakes:

• Using multiple arguments instead of an array
• Using logical operators inside $nin
• Missing square brackets around values

3. Given the collection products with documents:

{ name: 'Pen', category: 'stationery' }

{ name: 'Apple', category: 'fruit' }

{ name: 'Notebook', category: 'stationery' }

{ name: 'Carrot', category: 'vegetable' }

What will be the result of this query?

db.products.find({ category: { $nin: ['fruit', 'vegetable'] } })

medium

A. All documents returned

B. [{ name: 'Apple', category: 'fruit' }, { name: 'Carrot', category: 'vegetable' }]

C. [] (empty array)

D. [{ name: 'Pen', category: 'stationery' }, { name: 'Notebook', category: 'stationery' }]

Solution

1. Step 1: Understand the $nin filter

   The query excludes documents where category is 'fruit' or 'vegetable'.

2. Step 2: Identify matching documents

   Documents with category 'stationery' are not in ['fruit', 'vegetable'], so they match. 'Pen' and 'Notebook' have 'stationery'.

3. Final Answer:

   [{ name: 'Pen', category: 'stationery' }, { name: 'Notebook', category: 'stationery' }] -> Option D

4. Quick Check:

   Exclude 'fruit' and 'vegetable' = stationery items only [OK]

Hint: Exclude unwanted categories with $nin array [OK]

Common Mistakes:

• Confusing $nin with $in
• Expecting documents with excluded categories
• Assuming empty result when some match

4. You wrote this query to exclude users with roles 'admin' or 'moderator':

db.users.find({ role: { $nin: 'admin', 'moderator' } })

But it throws an error. What is the problem?

medium

A. The $nin operator requires an array, not multiple arguments

B. The field name role is misspelled

C. The query should use $in instead of $nin

D. MongoDB does not support $nin operator

Solution

1. Step 1: Check $nin syntax

   $nin expects a single array argument listing values to exclude.

2. Step 2: Identify the error in the query

   The query passes two separate string arguments instead of one array, causing syntax error.

3. Final Answer:

   The $nin operator requires an array, not multiple arguments -> Option A

4. Quick Check:

   $nin needs array input = C [OK]

Hint: Always wrap values in an array for $nin [OK]

Common Mistakes:

• Passing multiple arguments instead of an array
• Using wrong operator $in by mistake
• Assuming $nin is unsupported

5. You have a collection orders with documents:

{ orderId: 1, status: 'shipped' }

{ orderId: 2, status: 'pending' }

{ orderId: 3, status: 'cancelled' }

{ orderId: 4, status: 'delivered' }

You want to find orders NOT in statuses 'pending' or 'cancelled' AND exclude orders with orderId 4. Which query correctly uses $nin to achieve this?

hard

A. { status: { $nin: ['pending', 'cancelled'] }, orderId: { $ne: 4 } }

B. { status: { $nin: ['pending', 'cancelled', 4] } }

C. { status: { $nin: ['pending', 'cancelled'] }, orderId: { $nin: [4] } }

D. { status: { $nin: ['pending', 'cancelled'] }, orderId: { $in: [4] } }

Solution

1. Step 1: Exclude statuses 'pending' and 'cancelled'

   Use { status: { $nin: ['pending', 'cancelled'] } } to exclude these statuses.

2. Step 2: Exclude orderId 4 using $nin

   Use { orderId: { $nin: [4] } } to exclude orderId 4 as well.

3. Step 3: Combine conditions

   Both conditions together filter out unwanted statuses and orderId 4.

4. Final Answer:

   { status: { $nin: ['pending', 'cancelled'] }, orderId: { $nin: [4] } } -> Option C

5. Quick Check:

   Use $nin on both fields = A [OK]

Hint: Use separate $nin for each field to exclude multiple sets [OK]

Common Mistakes:

• Putting orderId inside status array
• Using $ne instead of $nin for multiple values
• Using $in to exclude values