NoSQL database types (document, key-value, column, graph) in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the NoSQL database type by data structure

Which NoSQL database type is best suited for storing data as flexible JSON-like documents that can vary in structure?

A Key-value store

B Graph database

C Document store

D Column store

💡 Hint

Think about a database that stores data as self-contained objects with fields and values.

Document stores save data as documents, often JSON or BSON, allowing flexible and nested data structures.

🧠 Conceptual

intermediate

Best NoSQL type for relationships between entities

Which NoSQL database type is designed to efficiently manage and query complex relationships between entities using nodes and edges?

A Column store

B Graph database

C Key-value store

D Document store

💡 Hint

Think about social networks or recommendation systems that need to explore connections.

Graph databases represent data as nodes and edges, making relationship queries fast and intuitive.

❓ query_result

advanced

Output of a key-value store query

Given a key-value store with pairs: {"user1": "Alice", "user2": "Bob", "user3": "Carol"}, what is the result of querying the key "user2"?

A "Bob"

B "user2"

C null

D Error: key not found

💡 Hint

Key-value stores return the value for the exact key queried.

Querying "user2" returns its associated value "Bob".

📋 Factual

advanced

Correct query to retrieve a column family in a column store

Which of the following is a valid CQL (Cassandra Query Language) statement to select all rows from the column family 'users'?

A SELECT * FROM users;

B FETCH ALL FROM users;

C GET * FROM users;

D READ * users;

💡 Hint

CQL syntax is similar to SQL for selecting data.

The correct syntax to select all rows is SELECT * FROM users;.

❓ optimization

expert

Optimizing queries in a graph database

Which approach best improves query performance when searching for shortest paths between nodes in a large graph database?

A Increase the number of columns in the column family storing nodes

B Store graph data as JSON documents for faster access

C Convert graph data into key-value pairs for quick lookup

D Use indexes on node properties and limit traversal depth

💡 Hint

Indexes help find starting points quickly and limiting depth reduces search space.

Indexes on node properties speed up locating nodes, and limiting traversal depth reduces computation for shortest path queries.

Practice

1. Which NoSQL database type is best suited for storing data as JSON-like documents with flexible schemas?

easy

A. Graph database

B. Document database

C. Column database

D. Key-value database

Solution

1. Step 1: Understand document database structure

   Document databases store data as documents, often JSON-like, allowing flexible and nested data.

2. Step 2: Compare with other NoSQL types

   Key-value stores use simple key-value pairs, column stores organize data by columns, and graph databases focus on relationships.

3. Final Answer:

   Document database -> Option B

4. Quick Check:

   Flexible JSON-like storage = Document database [OK]

Hint: JSON-like flexible data means document DB [OK]

Common Mistakes:

• Confusing key-value with document stores
• Thinking column stores handle JSON
• Assuming graph DB stores documents

2. Which of the following is the correct way to describe a key-value store?

easy

A. Stores data as nested JSON documents

B. Stores data as interconnected nodes and edges

C. Stores data in tables with rows and columns

D. Stores data as simple pairs of keys and values

Solution

1. Step 1: Define key-value store

   Key-value stores save data as pairs: a unique key and its associated value.

2. Step 2: Eliminate other options

   Nodes and edges describe graph DB, tables describe relational or column DB, nested JSON describes document DB.

3. Final Answer:

   Stores data as simple pairs of keys and values -> Option D

4. Quick Check:

   Key-value = key and value pairs [OK]

Hint: Key-value means simple pairs, not complex structures [OK]

Common Mistakes:

• Mixing graph DB with key-value store
• Confusing column DB with key-value
• Thinking document DB is key-value

3. Given a graph database storing people and their friendships, which query result would you expect from a query asking for all friends of 'Alice'?

medium

A. A set of nodes connected to 'Alice' by edges labeled 'friend'

B. A table with columns for friend names and ages

C. A list of key-value pairs with friend names

D. A JSON document containing Alice's profile

Solution

1. Step 1: Understand graph database query

   Graph DB queries return nodes and edges; friends of Alice are nodes connected by 'friend' edges.

2. Step 2: Compare expected outputs

   Key-value pairs or tables are not typical graph DB outputs; JSON document is for document DB.

3. Final Answer:

   A set of nodes connected to 'Alice' by edges labeled 'friend' -> Option A

4. Quick Check:

   Graph DB returns connected nodes and edges [OK]

Hint: Graph DB queries return nodes and edges, not tables or JSON [OK]

Common Mistakes:

• Expecting tabular output from graph DB
• Confusing document DB JSON with graph DB output
• Thinking key-value pairs represent graph edges

4. You wrote a query to retrieve data from a column-family NoSQL database but got an error. Which mistake likely caused this?

medium

A. Using nested JSON documents in the query

B. Querying nodes and edges instead of tables

C. Trying to access data by key only without specifying column family

D. Using key-value pairs without keys

Solution

1. Step 1: Understand column-family DB query requirements

   Column-family DBs require specifying column families to access data properly.

2. Step 2: Identify error cause

   Accessing data by key alone without column family causes errors; other options relate to different DB types or invalid syntax.

3. Final Answer:

   Trying to access data by key only without specifying column family -> Option C

4. Quick Check:

   Column DB needs column family in queries [OK]

Hint: Column DB queries must specify column family [OK]

Common Mistakes:

• Using document DB JSON syntax in column DB
• Ignoring column family in queries
• Confusing graph DB queries with column DB

5. You need to design a social network app that stores users, their posts, and complex friend relationships with recommendations. Which NoSQL database type should you choose and why?

hard

A. Graph database, because it efficiently manages complex relationships

B. Key-value database, because it is fastest for any data

C. Document database, because it handles nested posts well

D. Column database, because it stores large tables efficiently

Solution

1. Step 1: Analyze app data needs

   The app needs to store users, posts, and complex friend relationships with recommendations.

2. Step 2: Match database type to needs

   Graph DBs excel at managing complex relationships and traversals, ideal for social networks.

3. Step 3: Evaluate other options

   Document DB handles nested data but less efficient for relationships; key-value is simple but not relationship-focused; column DB is for wide tables, not relationships.

4. Final Answer:

   Graph database, because it efficiently manages complex relationships -> Option A

5. Quick Check:

   Complex relationships = Graph DB [OK]

Hint: Complex relationships? Choose graph DB [OK]

Common Mistakes:

• Choosing document DB for relationship-heavy data
• Assuming key-value is best for all speed needs
• Ignoring graph DB strengths in relationships