Tables vs collections thinking in MongoDB - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the difference between tables and collections

In MongoDB, which statement best describes the difference between a table in SQL and a collection in MongoDB?

A A table and a collection are exactly the same, both store data in fixed schemas with rows and columns.

B A table stores data as JSON documents, while a collection stores data in rows and columns.

C A table stores data in rows and columns with a fixed schema, while a collection stores documents that can have varying fields and structures.

D A table is used only for temporary data, while a collection stores permanent data.

💡 Hint

Think about how flexible the data structure is in MongoDB compared to SQL.

Tables in SQL databases have fixed schemas with rows and columns. Collections in MongoDB store documents that can have different fields and structures, allowing more flexibility.

❓ query_result

intermediate

Query result difference between SQL and MongoDB

Given a SQL table users with columns id and name, and a MongoDB collection users with documents having fields _id and name, what would be the result of the following MongoDB query?

db.users.find({ name: "Alice" })

Assuming the collection has these documents:

[{ _id: 1, name: "Alice" }, { _id: 2, name: "Bob" }, { _id: 3, name: "Alice", age: 30 }]

A []

B [{ _id: 1, name: "Alice" }, { _id: 3, name: "Alice", age: 30 }]

C [{ _id: 2, name: "Bob" }]

D [{ id: 1, name: "Alice" }]

💡 Hint

The query filters documents where the name field equals "Alice".

The query returns all documents where the name field is "Alice". Both documents with _id 1 and 3 match, including the one with an extra age field.

📝 Syntax

advanced

Identify the invalid MongoDB collection creation syntax

Which of the following commands will cause a syntax error when creating a collection in MongoDB?

A db.createCollection("orders", { capped: true, size: 5242880 })

B db.createCollection("products")

C db.products.insertOne({ name: "Book", price: 12.99 })

D db.createCollection(products)

💡 Hint

Look for missing quotes or incorrect argument types.

Option D is invalid because the collection name must be a string enclosed in quotes. Without quotes, it causes a syntax error.

❓ optimization

advanced

Optimizing data retrieval in collections vs tables

You have a large SQL table and a MongoDB collection both storing user data. Which approach is generally faster for retrieving a user by their unique ID?

A MongoDB's default _id index provides fast retrieval similar to SQL primary key indexes, so both are generally equally fast.

B MongoDB collections do not support indexes, so SQL tables are always faster.

C Using an indexed primary key in SQL tables is generally faster than MongoDB's default _id index.

D MongoDB requires scanning the entire collection, so SQL tables are faster for unique ID lookups.

💡 Hint

Think about how both systems index unique identifiers.

Both SQL tables and MongoDB collections use indexes on unique IDs (_id in MongoDB, primary key in SQL) to quickly retrieve data, making their performance similar for such queries.

🔧 Debug

expert

Debugging schema design misunderstanding between tables and collections

A developer tries to enforce a fixed schema in a MongoDB collection by creating a table-like structure with all fields mandatory. Which problem will most likely occur?

A MongoDB allows flexible schemas, so documents missing some fields will be accepted, leading to inconsistent data.

B MongoDB will reject any document that does not have all fields, causing frequent insertion errors.

C MongoDB automatically fills missing fields with null values to enforce the schema.

D MongoDB converts the collection into a SQL table to enforce the schema.

💡 Hint

Consider MongoDB's schema flexibility and validation behavior by default.

MongoDB collections are schema-less by default, so documents missing some fields are accepted unless explicit validation rules are set. This can lead to inconsistent data if a fixed schema is expected but not enforced.

Practice

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

easy

A. A collection cannot store nested data.

B. A collection stores flexible documents without fixed columns.

C. A collection is organized strictly in rows and columns.

D. A collection requires fixed columns and strict schema.

Solution

1. Step 1: Understand MongoDB collections

   MongoDB collections store documents that can have different fields and structures.

2. Step 2: Compare with SQL tables

   SQL tables require fixed columns and rows, unlike collections.

3. Final Answer:

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

4. Quick Check:

   Collections = flexible documents [OK]

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

Common Mistakes:

• Thinking collections require fixed columns
• Assuming collections store data in rows and columns
• Believing collections cannot have nested data

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

easy

A. CREATE TABLE users;

B. CREATE COLLECTION users;

C. db.createCollection('users');

D. db.users.create();

Solution

1. Step 1: Recall MongoDB syntax for creating collections

   MongoDB uses db.createCollection('name') to create collections.

2. Step 2: Check other options

   Options A, B, and D use invalid MongoDB syntax.

3. Final Answer:

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

4. Quick Check:

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

Hint: Use db.createCollection('name') in MongoDB [OK]

Common Mistakes:

• Using SQL CREATE TABLE syntax in MongoDB
• Trying to call create() on collection object
• Using CREATE COLLECTION which is invalid

3. Given a MongoDB collection products with documents like { name: 'Pen', price: 1.5 } and { name: 'Notebook', price: 3 }, what will db.products.find({ price: { $gt: 2 } }) return?

medium

A. All products with price greater than 2

B. All products with price less than 2

C. All products with price equal to 2

D. Syntax error

Solution

1. Step 1: Understand the query filter

   The filter { price: { $gt: 2 } } selects documents where price is greater than 2.

2. Step 2: Apply filter to example data

   Only the document with price 3 matches; price 1.5 does not.

3. Final Answer:

   All products with price greater than 2 -> Option A

4. Quick Check:

   $gt means greater than [OK]

Hint: Remember $gt means greater than in MongoDB queries [OK]

Common Mistakes:

• Confusing $gt with $lt
• Thinking it returns products with price less than 2
• Assuming syntax error due to $gt usage

4. You tried to insert a document into a MongoDB collection with db.orders.insert({id: 1, item: 'Book'}) but got a deprecation warning. What is the likely cause?

medium

A. The insert method is deprecated; use insertOne or insertMany instead.

B. MongoDB requires documents to have a field named _id, not id.

C. The collection name must be plural, so 'orders' is invalid.

D. Documents cannot have string values in MongoDB.

Solution

1. Step 1: Check MongoDB insert method usage

   The insert() method is deprecated in modern MongoDB versions.

2. Step 2: Use correct insertion methods

   Use insertOne() or insertMany() to insert documents.

3. Final Answer:

   The insert method is deprecated; use insertOne or insertMany instead. -> Option A

4. Quick Check:

   Use insertOne/insertMany, not insert() [OK]

Hint: Use insertOne or insertMany, not insert() [OK]

Common Mistakes:

• Thinking _id field is mandatory to insert
• Believing collection names must be plural
• Assuming string values are invalid in documents

5. You want to store user profiles where each user can have different sets of contact methods (email, phone, social media). Which is the best approach in MongoDB?

hard

A. Create separate tables for each contact method and join them.

B. Store all contact methods as a single string field.

C. Use a fixed schema collection with all possible contact fields, leaving some empty.

D. Use a single collection with flexible documents holding different contact fields.

Solution

1. Step 1: Consider MongoDB's flexible schema

   MongoDB collections allow documents to have different fields, perfect for varying contact methods.

2. Step 2: Compare with relational approach

   Relational tables require joins and fixed schemas, less flexible for this use case.

3. Final Answer:

   Use a single collection with flexible documents holding different contact fields. -> Option D

4. Quick Check:

   Flexible documents fit varying user data best [OK]

Hint: Flexible documents handle varied user data best [OK]

Common Mistakes:

• Trying to use fixed schema for varying data
• Using multiple tables and joins in MongoDB
• Storing complex data as a single string