$ne for not equal in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents where age is not 30

Given a collection users with documents containing an age field, which query returns all users whose age is not 30?

MongoDB

db.users.find({ age: { $ne: 30 } })

A Returns users with age greater than 30

B Returns only users with age 30

C Returns all users regardless of age

D Returns all users except those with age 30

💡 Hint

Use $ne to find values not equal to a specific number.

The $ne operator selects documents where the value of the field is not equal to the specified value. Here, { age: { $ne: 30 } } finds all users whose age is anything but 30.

🧠 Conceptual

intermediate

Understanding $ne with null values

What does the query db.collection.find({ field: { $ne: null } }) return?

A Documents where <code>field</code> is null

B Documents where <code>field</code> does not exist or exists and is not null

C Documents where <code>field</code> does not exist

D Documents where <code>field</code> is either null or does not exist

💡 Hint

Think about how MongoDB treats null and missing fields.

The $ne: null condition matches documents where the field does not exist or its value is not null. It excludes documents where the field exists and is null.

📝 Syntax

advanced

Identify the syntax error in $ne usage

Which of the following queries has a syntax error?

A db.items.find({ price: { $ne: null } })

B db.items.find({ price: { $ne: 100 } })

C db.items.find({ price: $ne: 100 })

D db.items.find({ price: { $ne: '100' } })

💡 Hint

Check the correct way to use operators inside query objects.

Option C is invalid because the operator $ne must be inside an object as a key, not used directly as a key-value pair without braces.

❓ optimization

advanced

Optimizing queries with $ne

Which query is more efficient to find documents where status is not 'active'?

A db.records.find({ status: { $ne: 'active' } })

B db.records.find({ status: { $not: { $eq: 'active' } } })

C db.records.find({ status: { $nin: ['active'] } })

D db.records.find({ status: { $exists: true, $ne: 'active' } })

💡 Hint

Consider the simplest operator that directly expresses the condition.

Option A uses $ne directly, which is straightforward and efficient. Options A and C are more complex and may be less efficient. Option A adds an unnecessary $exists check.

🔧 Debug

expert

Why does this $ne query return unexpected results?

Consider the query db.products.find({ category: { $ne: 'electronics' } }). It returns documents where category is missing. Why does this happen?

A Because $ne matches documents where the field is not equal or the field does not exist

B Because $ne only matches documents where the field exists and is not equal

C Because MongoDB treats missing fields as null and excludes them

D Because the query syntax is incorrect and matches all documents

💡 Hint

Think about how MongoDB treats missing fields with $ne.

The $ne operator matches documents where the field value is not equal to the specified value or where the field does not exist at all. This is why documents missing the category field are included.

Practice

1.

What does the $ne operator do in MongoDB queries?

easy

A. Finds documents where a field is NOT equal to a specified value

B. Finds documents where a field is equal to a specified value

C. Finds documents where a field is greater than a specified value

D. Finds documents where a field is less than a specified value

Solution

1. Step 1: Understand the purpose of $ne

   The $ne operator is used to filter documents where a field's value is not equal to the given value.

2. Step 2: Compare with other operators

   Other operators like $eq check for equality, but $ne specifically excludes matching values.

3. Final Answer:

   Finds documents where a field is NOT equal to a specified value -> Option A

4. Quick Check:

   $ne = Not Equal [OK]

Hint: Remember: $ne means 'not equal' in queries [OK]

Common Mistakes:

• Confusing $ne with $eq
• Thinking $ne checks for greater or less than
• Using $ne without a value

2.

Which of the following is the correct syntax to find documents where the field status is NOT equal to "active"?

{ status: { ? } }

easy

A. { $not: "active" }

B. { $eq: "active" }

C. { $ne: "active" }

D. { $neq: "active" }

Solution

1. Step 1: Recall correct operator syntax

   The $ne operator is used with the syntax: { field: { $ne: value } } to find documents where the field is not equal to the value.

2. Step 2: Check other options for correctness

   $eq checks equality, $not is used differently, and $neq is not a valid MongoDB operator.

3. Final Answer:

   { $ne: "active" } -> Option C

4. Quick Check:

   Correct syntax uses $ne [OK]

Hint: Use { field: { $ne: value } } for not equal queries [OK]

Common Mistakes:

• Using $neq instead of $ne
• Confusing $not with $ne
• Missing curly braces around $ne

3.

Given the collection users with documents:

[{ "name": "Alice", "age": 25 }, { "name": "Bob", "age": 30 }, { "name": "Carol", "age": 25 }]

What will be the result of the query db.users.find({ age: { $ne: 25 } })?

medium

A. [{ "name": "Bob", "age": 30 }]

B. [{ "name": "Alice", "age": 25 }, { "name": "Carol", "age": 25 }]

C. [] (empty array)

D. All documents in the collection

Solution

1. Step 1: Understand the query condition

   The query { age: { $ne: 25 } } finds documents where the age is NOT equal to 25.

2. Step 2: Check each document's age

   Alice and Carol have age 25, so they are excluded. Bob has age 30, so he matches.

3. Final Answer:

   [{ "name": "Bob", "age": 30 }] -> Option A

4. Quick Check:

   Only age != 25 returns Bob [OK]

Hint: Exclude matching values with $ne to get others [OK]

Common Mistakes:

• Including documents with age 25
• Returning all documents by mistake
• Confusing $ne with $eq

4.

Identify the error in this MongoDB query to find documents where category is NOT equal to "books":

db.collection.find({ category: { $ne: books } })

medium

A. The query is correct as is

B. Incorrect operator, should use $neq instead of $ne

C. The field name should be in quotes

D. Missing quotes around the string value "books"

Solution

1. Step 1: Check the value type in the query

   The value "books" is a string and must be enclosed in quotes in MongoDB queries.

2. Step 2: Verify operator and field name

   The operator $ne is correct, and field names do not require quotes unless special characters are present.

3. Final Answer:

   Missing quotes around the string value "books" -> Option D

4. Quick Check:

   String values need quotes [OK]

Hint: Always quote string values in queries [OK]

Common Mistakes:

• Leaving string values unquoted
• Using $neq instead of $ne
• Quoting field names unnecessarily

5.

You have a collection products with documents containing type and price. You want to find all products that are NOT of type "electronics" and have a price NOT equal to 100. Which query correctly uses $ne to achieve this?

hard

A. { $ne: { type: "electronics", price: 100 } }

B. { type: { $ne: "electronics" }, price: { $ne: 100 } }

C. { type: { $ne: "electronics" } || price: { $ne: 100 } }

D. { type: { $not: "electronics" }, price: { $not: 100 } }

Solution

1. Step 1: Use $ne on each field separately

   To find documents where type is not "electronics" and price is not 100, apply $ne to each field individually.

2. Step 2: Check query syntax correctness

   { type: { $ne: "electronics" }, price: { $ne: 100 } } correctly uses { type: { $ne: "electronics" }, price: { $ne: 100 } }. { $ne: { type: "electronics", price: 100 } } misuses $ne on an object, { type: { $ne: "electronics" } || price: { $ne: 100 } } uses invalid syntax with ||, and { type: { $not: "electronics" }, price: { $not: 100 } } uses $not incorrectly.

3. Final Answer:

   { type: { $ne: "electronics" }, price: { $ne: 100 } } -> Option B

4. Quick Check:

   Apply $ne to each field separately [OK]

Hint: Use $ne on each field inside the query object [OK]

Common Mistakes:

• Using $ne on an object instead of fields
• Using logical OR (||) inside query object incorrectly
• Confusing $not with $ne