$or operator behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Understanding $or with multiple conditions

Given a collection products with documents:

{ "name": "Pen", "price": 5, "stock": 100 }

{ "name": "Notebook", "price": 15, "stock": 0 }

{ "name": "Eraser", "price": 3, "stock": 50 }

What documents will be returned by this query?

{ $or: [ { price: { $lt: 5 } }, { stock: { $eq: 0 } } ] }

A Documents with name "Pen" and "Eraser"

B Documents with name "Pen" and "Notebook"

C Documents with name "Notebook" and "Eraser"

D All three documents

💡 Hint

Remember $or returns documents matching at least one condition.

The query matches documents where price is less than 5 OR stock equals 0. "Eraser" has price 3 (<5), "Notebook" has stock 0, "Pen" matches neither.

🧠 Conceptual

intermediate

Behavior of $or with empty array

What happens if you run a MongoDB query with { $or: [] }?

A Returns all documents in the collection

B Returns no documents

C Throws a syntax error

D Returns documents matching all conditions

💡 Hint

Think about what it means to match any condition when there are none.

An empty $or means no conditions to satisfy, so no documents match. It returns zero documents.

📝 Syntax

advanced

Identify the invalid $or query syntax

Which of the following MongoDB queries using $or is syntactically invalid?

A { $or: { age: { $lt: 20 }, city: "NY" } }

B { $or: [ { age: { $gt: 30 } }, { name: "Alice" } ] }

C { $or: [ { score: { $gte: 90 } }, { grade: "A" } ] }

D { $or: [ { active: true }, { verified: false } ] }

💡 Hint

$or expects an array of conditions, not an object.

Option A uses an object instead of an array for $or, which is invalid syntax.

❓ optimization

advanced

Optimizing $or queries with indexes

You have a collection with indexes on status and priority. Which $or query will best use these indexes?

A { $or: [ { status: "open" }, { priority: { $gt: 5 } } ] }

B { $or: [ { status: "open", priority: { $gt: 5 } } ] }

C { $or: [ { status: "open" }, { category: "urgent" } ] }

D { $or: [ { category: "urgent" }, { priority: { $gt: 5 } } ] }

💡 Hint

Indexes help when the fields in the query match the indexed fields.

Option A uses $or with conditions on indexed fields status and priority, allowing index use for both parts.

🔧 Debug

expert

Why does this $or query return unexpected results?

Consider this query on a collection of users:

{ $or: [ { age: { $gt: 25 } }, { age: { $lt: 20 } } ] }

It returns all users, including those with age 22. Why?

A Because the $or conditions cover all ages except 20 to 25, so 22 is excluded

B Because the query matches users with age greater than 25 OR less than 20, so age 22 does not match and should not be returned

C Because the query matches users with age greater than 25 OR less than 20, so age 22 does not match either condition

D Because the query matches users with age greater than 25 OR less than 20, but the field age is stored as a string causing unexpected matches

💡 Hint

Check the data type of the age field in the documents.

If age is stored as a string, comparisons like $gt and $lt behave lexicographically, causing unexpected matches like '22' being treated differently.