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.

Practice

1. Which of the following best describes a MongoDB collection compared to a SQL table?

easy

A. A collection requires a strict schema like SQL tables.

B. A collection stores data in fixed rows and columns.

C. A collection stores flexible documents without fixed columns.

D. A collection cannot store multiple records.

Solution

1. Step 1: Understand SQL tables

   SQL tables store data in rows and columns with a fixed schema, meaning each row has the same columns.

2. Step 2: Understand MongoDB collections

   MongoDB collections store documents that can have different fields and structures, so they are flexible and schema-less.

3. Final Answer:

   A collection stores flexible documents without fixed columns. -> Option C

4. Quick Check:

   Collections = flexible documents [OK]

Hint: Collections are flexible; tables have fixed columns [OK]

Common Mistakes:

• Thinking collections have fixed columns like tables
• Assuming collections require strict schemas
• Believing collections cannot hold multiple documents

2. Which of the following is the correct way to create a collection named users in MongoDB?

easy

A. db.createCollection('users');

B. db.users.create();

C. CREATE TABLE users;

D. CREATE COLLECTION users;

Solution

1. Step 1: Recall MongoDB syntax for creating collections

   MongoDB uses the method db.createCollection('name') to create a collection explicitly.

2. Step 2: Check other options

   CREATE COLLECTION users; and C use SQL syntax, which is invalid in MongoDB. db.users.create(); is not a valid MongoDB command.

3. Final Answer:

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

4. Quick Check:

   MongoDB collection creation = db.createCollection() [OK]

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

Common Mistakes:

• Using SQL syntax like CREATE TABLE
• Trying to call create() on collection name
• Omitting quotes around collection name

3. Given a MongoDB collection products with documents:

{"name": "Pen", "price": 1.5}

{"name": "Notebook", "price": 3.0, "color": "blue"}

What will db.products.find({}) return?

medium

A. All documents including fields like name, price, and color if present.

B. Only documents with the price field.

C. Only documents with the color field.

D. An error because documents have different fields.

Solution

1. Step 1: Understand find({}) query

   The query find({}) returns all documents in the collection regardless of their fields.

2. Step 2: Check document fields

   Documents can have different fields in MongoDB collections; this does not cause errors or filtering.

3. Final Answer:

   All documents including fields like name, price, and color if present. -> Option A

4. Quick Check:

   find({}) returns all documents [OK]

Hint: find({}) returns all documents regardless of fields [OK]

Common Mistakes:

• Thinking find({}) filters by fields
• Expecting errors due to different document structures
• Assuming only documents with certain fields are returned

4. You tried to insert documents with different fields into a MongoDB collection but got an error. What is the most likely cause?

medium

A. MongoDB collections require all documents to have the same fields.

B. MongoDB does not allow inserting multiple documents.

C. You forgot to create the collection before inserting.

D. You used SQL INSERT syntax instead of MongoDB insert methods.

Solution

1. Step 1: Recall MongoDB insert syntax

   MongoDB uses insertOne() or insertMany() methods, not SQL INSERT statements.

2. Step 2: Understand document flexibility

   MongoDB collections allow documents with different fields; this does not cause errors.

3. Final Answer:

   You used SQL INSERT syntax instead of MongoDB insert methods. -> Option D

4. Quick Check:

   MongoDB insert = insertOne()/insertMany() [OK]

Hint: Use MongoDB insertOne()/insertMany(), not SQL INSERT [OK]

Common Mistakes:

• Assuming MongoDB requires fixed schema
• Not creating collection first (MongoDB auto-creates collections)
• Believing MongoDB forbids multiple inserts

5. You want to migrate a SQL table with fixed columns to MongoDB. Which approach best fits MongoDB's collection model?

hard

A. Keep the exact SQL schema and enforce it strictly in MongoDB.

B. Store each row as a document with the same fields, but allow optional fields for future flexibility.

C. Convert the table into multiple collections, each with one column as documents.

D. Store all rows as a single large document with arrays for each column.

Solution

1. Step 1: Understand MongoDB document flexibility

   MongoDB collections store documents that can have different fields, so keeping the same fields but allowing optional ones fits well.

2. Step 2: Evaluate other options

   Convert the table into multiple collections, each with one column as documents. breaks data into many collections unnecessarily. Keep the exact SQL schema and enforce it strictly in MongoDB. tries to enforce strict schema, which MongoDB does not require. Store all rows as a single large document with arrays for each column. stores all data in one document, which is inefficient and not recommended.

3. Final Answer:

   Store each row as a document with the same fields, but allow optional fields for future flexibility. -> Option B

4. Quick Check:

   MongoDB collections = flexible documents per row [OK]

Hint: Migrate rows as flexible documents with optional fields [OK]

Common Mistakes:

• Trying to enforce strict SQL schema in MongoDB
• Splitting columns into separate collections
• Storing all rows in one big document