Database and collection creation in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this MongoDB command?

Consider the following MongoDB shell command:

use myNewDB

What will be the output after running this command?

MongoDB

use myNewDB

A "database myNewDB dropped"

B "error: database does not exist"

C "switched to db myNewDB"

D "created database myNewDB"

💡 Hint

The 'use' command switches the current database context.

The 'use' command in MongoDB switches the shell to the specified database. If the database does not exist yet, it will be created when you first store data in it. The shell outputs 'switched to db '.

❓ query_result

intermediate

What happens when you create a collection explicitly?

Given the command:

db.createCollection('users')

What is the result of running this command in MongoDB shell?

MongoDB

db.createCollection('users')

A {"ok": 1}

B {"ok": 0}

C null

D Error: collection already exists

💡 Hint

Creating a collection explicitly returns a success status.

The createCollection command returns an object with 'ok' set to 1 on success. If the collection already exists, it throws an error.

📝 Syntax

advanced

Which command correctly creates a capped collection named 'logs'?

You want to create a capped collection named 'logs' with a maximum size of 1MB. Which command is correct?

A db.createCollection('logs', {size: 1048576, capped: false})

B db.createCollection('logs', {capped: true, size: 1048576})

C db.createCollection('logs', {capped: true, max: 1048576})

D db.createCollection('logs', {maxSize: 1048576, capped: true})

💡 Hint

The 'capped' option must be true and 'size' specifies the max bytes.

To create a capped collection, set 'capped' to true and specify 'size' in bytes. 'max' is for max documents, not size. 'maxSize' is not a valid option.

🧠 Conceptual

advanced

Why does MongoDB create a database only after inserting data?

In MongoDB, when you run use newDB, the database is not created immediately. Why is this behavior designed this way?

A Because databases are created automatically when the server starts.

B Because the 'use' command is only for switching databases and cannot create them.

C Because MongoDB requires a restart to create new databases.

D Because MongoDB creates databases lazily only when data is stored to save disk space.

💡 Hint

Think about resource usage and efficiency.

MongoDB delays creating a database until data is inserted to avoid creating empty databases and wasting disk space.

🔧 Debug

expert

Identify the error in this collection creation command

What error will this command produce?

db.createCollection('events', {capped: true, size: '1000'})

Note: size is given as a string.

MongoDB

db.createCollection('events', {capped: true, size: '1000'})

A MongoServerError: capped collections require size in bytes

B SyntaxError: unexpected string

C No error, collection created successfully

D TypeError: size must be a number

💡 Hint

Check the data type required for the size option.

The 'size' option must be a number representing bytes. Passing a string causes a MongoServerError indicating the size must be numeric.

Practice

1. What does the command use myDatabase do in MongoDB?

easy

A. It deletes the database named 'myDatabase'.

B. It creates a collection named 'myDatabase'.

C. It switches to the database named 'myDatabase' or creates it if it doesn't exist.

D. It lists all collections in the current database.

Solution

1. Step 1: Understand the use command

   The use command in MongoDB switches the current database context to the specified database.

2. Step 2: Behavior when database does not exist

   If the specified database does not exist, MongoDB creates it when you first store data in it.

3. Final Answer:

   It switches to the database named 'myDatabase' or creates it if it doesn't exist. -> Option C

4. Quick Check:

   use databaseName switches or creates database [OK]

Hint: Remember: use switches or creates database [OK]

Common Mistakes:

• Thinking use creates collections
• Confusing database and collection commands
• Assuming use deletes databases

2. Which of the following is the correct syntax to create a new collection named users in the current database?

easy

A. db.createCollection('users')

B. create collection users

C. db.users.create()

D. use users

Solution

1. Step 1: Identify the MongoDB command for collection creation

   The correct MongoDB command to create a collection is db.createCollection('collectionName').

2. Step 2: Match the syntax with the options

   db.createCollection('users') matches the correct syntax exactly for creating a collection named 'users'.

3. Final Answer:

   db.createCollection('users') -> Option A

4. Quick Check:

   Use db.createCollection('name') to create collections [OK]

Hint: Use db.createCollection('name') to create collections [OK]

Common Mistakes:

• Using SQL-like syntax such as 'create collection users'
• Trying to create collection with db.users.create()
• Using use users to create collection

3. What will be the result of running these commands in order?

use shopDB
db.createCollection('products')
db.getCollectionNames()

medium

A. Error: createCollection not found

B. ['shopDB']

C. []

D. ['products']

Solution

1. Step 1: Switch to or create the database 'shopDB'

   The use shopDB command switches to the database named 'shopDB'. If it doesn't exist, it will be created when data is added.

2. Step 2: Create a collection named 'products'

   The command db.createCollection('products') creates a new collection called 'products' inside 'shopDB'.

3. Step 3: List collections in the current database

   db.getCollectionNames() returns an array of collection names in the current database. Since 'products' was created, it returns ['products'].

4. Final Answer:

   ['products'] -> Option D

5. Quick Check:

   Created collection appears in db.getCollectionNames() [OK]

Hint: Created collections appear in db.getCollectionNames() [OK]

Common Mistakes:

• Expecting database name in collection list
• Assuming collections list is empty after creation
• Confusing collection names with database names

4. Identify the error in this MongoDB command sequence:

use testDB
db.createCollection(users)

medium

A. Missing quotes around collection name 'users'.

B. The use command is incorrect.

C. The collection name cannot be 'users'.

D. The command createCollection does not exist.

Solution

1. Step 1: Check syntax of createCollection command

   The collection name must be a string, so it requires quotes around it.

2. Step 2: Identify the error in the command

   The command db.createCollection(users) is missing quotes around 'users', causing a syntax error.

3. Final Answer:

   Missing quotes around collection name 'users'. -> Option A

4. Quick Check:

   Collection names must be strings in quotes [OK]

Hint: Always put collection names in quotes in createCollection [OK]

Common Mistakes:

• Omitting quotes around collection names
• Misusing use command syntax
• Thinking collection names cannot be common words

5. You want to create a new database called libraryDB and inside it a collection named books. Which sequence of commands will achieve this correctly?

hard

A. create database libraryDB
create collection books

B. use libraryDB
db.createCollection('books')

C. use books
db.createCollection('libraryDB')

D. db.createCollection('libraryDB.books')

Solution

1. Step 1: Switch to or create the database 'libraryDB'

   The command use libraryDB switches to the database named 'libraryDB'. If it doesn't exist, it will be created when data is added.

2. Step 2: Create the collection 'books' inside 'libraryDB'

   Using db.createCollection('books') creates the collection named 'books' in the current database context.

3. Final Answer:

   use libraryDB db.createCollection('books') -> Option B

4. Quick Check:

   Switch database then create collection [OK]

Hint: Switch database first, then create collection inside it [OK]

Common Mistakes:

• Trying to create database and collection in one command
• Using SQL syntax like 'create database'
• Confusing collection and database names