What is MongoDB - Practice Questions & Exercises

Challenge - 5 Problems

🧠 Conceptual

intermediate

What type of database is MongoDB?

MongoDB is best described as which type of database?

A A document-oriented NoSQL database that stores data in JSON-like documents

B A relational database that uses tables and rows

C A graph database that stores data as nodes and edges

D A key-value store that only supports simple key-value pairs

💡 Hint

Think about how MongoDB stores data compared to traditional databases.

MongoDB stores data as flexible JSON-like documents, making it a document-oriented NoSQL database, unlike relational databases that use tables.

🧠 Conceptual

intermediate

Which feature is a key advantage of MongoDB?

What is a main advantage of using MongoDB over traditional relational databases?

A It requires a fixed schema before storing data

B It cannot scale horizontally across servers

C It only supports SQL queries

D It supports flexible schemas allowing different documents to have different structures

💡 Hint

Think about how MongoDB handles data structure changes.

MongoDB allows flexible schemas, so documents in the same collection can have different fields and structures, unlike fixed schemas in relational databases.

❓ query_result

advanced

What is the output of this MongoDB query?

Given a collection 'users' with documents: {name: 'Alice', age: 30}, {name: 'Bob', age: 25}, what does this query return? db.users.find({age: {$gt: 27}})

MongoDB

db.users.find({age: {$gt: 27}})

A [{name: 'Alice', age: 30}]

B [{name: 'Bob', age: 25}]

C []

D SyntaxError

💡 Hint

Look for users with age greater than 27.

The query finds documents where age is greater than 27, so only Alice's document matches.

📝 Syntax

advanced

Which MongoDB query syntax correctly updates a user's age?

You want to update the age of the user named 'Bob' to 28. Which query is correct?

A db.users.modify({name: 'Bob'}, {$set: {age: 28}})

B db.users.update({name: 'Bob'}, {age: 28})

C db.users.updateOne({name: 'Bob'}, {$set: {age: 28}})

D db.users.updateOne({name: 'Bob'}, {age: 28})

💡 Hint

Remember the correct update operator and method name.

The correct syntax uses updateOne with a filter and the $set operator to update fields.

❓ optimization

expert

How to improve query performance in MongoDB?

You have a large collection and queries on the 'email' field are slow. What is the best way to improve query speed?

A Increase the RAM of the server without changing the database

B Create an index on the 'email' field

C Use $where with JavaScript to filter emails

D Store emails in a separate collection

💡 Hint

Indexes help databases find data faster.

Creating an index on the 'email' field allows MongoDB to quickly locate documents matching queries on that field.

Practice

1. What is MongoDB primarily used for?

easy

A. Compiling programming languages

B. Creating static web pages

C. Storing data as flexible documents inside collections

D. Designing user interfaces

Solution

1. Step 1: Understand MongoDB's data storage

   MongoDB stores data in a flexible, document-based format rather than tables.

2. Step 2: Identify the main use case

   This document storage is organized inside collections, making it easy to manage data.

3. Final Answer:

   Storing data as flexible documents inside collections -> Option C

4. Quick Check:

   MongoDB = flexible document storage [OK]

Hint: MongoDB stores data as documents, not tables [OK]

Common Mistakes:

• Confusing MongoDB with SQL databases
• Thinking MongoDB is for web design
• Assuming MongoDB compiles code

2. Which of the following is the correct way to insert a document into a MongoDB collection named users?

easy

A. insert into users values ('Alice', 30)

B. add users {name: 'Alice', age: 30}

C. INSERT INTO users (name, age) VALUES ('Alice', 30)

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

Solution

1. Step 1: Recognize MongoDB insert syntax

   MongoDB uses insertOne() method on a collection object to add a document.

2. Step 2: Compare options

   db.users.insertOne({name: 'Alice', age: 30}) uses correct MongoDB syntax; others use SQL or invalid commands.

3. Final Answer:

   db.users.insertOne({name: 'Alice', age: 30}) -> Option D

4. Quick Check:

   MongoDB insert = insertOne() method [OK]

Hint: MongoDB uses insertOne() to add documents [OK]

Common Mistakes:

• Using SQL insert syntax in MongoDB
• Missing the collection name before insertOne()
• Using invalid commands like 'add'

3. What will be the output of the following MongoDB query?

db.products.find({price: {$gt: 100}})

medium

A. All products with price greater than 100

B. Syntax error in query

C. All products with price equal to 100

D. All products with price less than 100

Solution

1. Step 1: Understand the query filter

   The query uses {$gt: 100} which means 'greater than 100'.

2. Step 2: Interpret the find() result

   The query returns all documents in products where the price field is greater than 100.

3. Final Answer:

   All products with price greater than 100 -> Option A

4. Quick Check:

   {price: {$gt: 100}} means price > 100 [OK]

Hint: {$gt: value} means greater than value [OK]

Common Mistakes:

• Confusing $gt with $lt
• Thinking it returns price equal to 100
• Assuming syntax error due to $gt

4. Identify the error in this MongoDB update command:

db.users.update({name: 'Bob'}, {age: 25})

medium

A. Missing $set operator to update fields

B. Collection name is incorrect

C. Query filter is invalid

D. Syntax is correct, no error

Solution

1. Step 1: Review update command syntax

   MongoDB requires using $set to update specific fields without replacing the whole document.

2. Step 2: Identify missing $set

   The command tries to update age directly, which replaces the whole document except for _id.

3. Final Answer:

   Missing $set operator to update fields -> Option A

4. Quick Check:

   Update needs $set for field changes [OK]

Hint: Use $set to update fields without replacing document [OK]

Common Mistakes:

• Forgetting $set causes document replacement
• Assuming update() auto-merges fields
• Confusing update() with insert()

5. You want to store user profiles where each user can have different fields like hobbies, address, or preferences. Why is MongoDB a good choice for this?

hard

A. Because MongoDB enforces a strict schema for all documents

B. Because MongoDB stores data as flexible documents allowing different fields

C. Because MongoDB requires all documents to have the same fields

D. Because MongoDB only supports fixed table columns

Solution

1. Step 1: Understand MongoDB's schema flexibility

   MongoDB allows documents in the same collection to have different fields and structures.

2. Step 2: Match flexibility to user profiles

   User profiles with varying fields fit well because MongoDB does not require a fixed schema.

3. Final Answer:

   Because MongoDB stores data as flexible documents allowing different fields -> Option B

4. Quick Check:

   MongoDB = flexible schema for varied data [OK]

Hint: MongoDB allows different fields per document [OK]

Common Mistakes:

• Thinking MongoDB requires fixed schemas
• Confusing MongoDB with relational databases
• Assuming all documents must match exactly