Collections vs tables mental model in MongoDB - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the basic difference between collections and tables

In MongoDB, what is the closest equivalent of a SQL table?

A A document

B A collection

C A field

D A database

💡 Hint

Think about where groups of similar data are stored.

A collection in MongoDB is similar to a table in SQL. Both store groups of related data entries.

❓ query_result

intermediate

Querying documents in a collection vs rows in a table

Given a MongoDB collection named users with documents containing name and age, which query returns all users older than 25?

MongoDB

db.users.find({ age: { $gt: 25 } })

A SELECT * FROM users WHERE age > 25;

B SELECT * FROM users WHERE age < 25;

C db.users.find({ age: { $lt: 25 } })

D db.users.find({ age: 25 })

💡 Hint

Look for the query that matches the condition 'age greater than 25'.

The MongoDB query db.users.find({ age: { $gt: 25 } }) matches the SQL query SELECT * FROM users WHERE age > 25; because both filter for age greater than 25.

📝 Syntax

advanced

Identifying the correct syntax for inserting data

Which MongoDB command correctly inserts a document with fields name and age into the users collection?

A db.users.insert({ 'name': 'Alice', 'age': 30 })

B INSERT INTO users (name, age) VALUES ('Alice', 30);

C db.users.insertOne({ name: 'Alice', age: 30 })

D db.users.add({ name: 'Alice', age: 30 })

💡 Hint

MongoDB uses JavaScript-like commands for inserting one document.

The correct MongoDB command to insert one document is insertOne(). Option C uses deprecated insert() which still works but is not recommended. Option C is SQL syntax. Option C is invalid.

❓ optimization

advanced

Choosing the best index for fast queries

You have a MongoDB collection orders with many documents. You often query orders by customerId. Which index improves query speed for db.orders.find({ customerId: 123 })?

A Create a text index on the <code>customerId</code> field

B Create an index on the <code>orderDate</code> field

C Create a unique index on the <code>orderId</code> field

D Create an index on the <code>customerId</code> field

💡 Hint

Think about which field is used in the query filter.

Indexing the customerId field speeds up queries filtering by that field. Indexing unrelated fields or using text indexes on numeric fields won't help.

🔧 Debug

expert

Diagnosing a query that returns no results

You run db.products.find({ price: { $gt: 100 } }) but get no results, even though you know some products have price over 100. What is the most likely reason?

A The <code>price</code> field is stored as a string, not a number

B The collection <code>products</code> does not exist

C The query syntax is incorrect

D The database is empty

💡 Hint

Think about how data types affect comparison operators.

If price is stored as a string, numeric comparison operators like $gt won't work as expected, causing no matches.