$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.