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

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the NoSQL database type from its description

Which NoSQL database type stores data as flexible, self-describing JSON-like documents allowing nested structures and varied fields per record?

A Document store

B Key-value store

C Column store

D Graph database

💡 Hint

Think about databases that store data in JSON or BSON format.

Document stores save data as documents, often JSON-like, which can have nested fields and flexible schemas.

🧠 Conceptual

intermediate

Match the NoSQL type to its main use case

Which NoSQL database type is best suited for storing and querying relationships between entities, such as social networks or recommendation systems?

A Column store

B Key-value store

C Document store

D Graph database

💡 Hint

Think about databases that model data as nodes and edges.

Graph databases specialize in storing entities and their relationships, making them ideal for social networks.

❓ query_result

advanced

Output of a key-value store retrieval

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

HLD

GET user2

A "Alice"

B "Bob"

C null

D "user2"

💡 Hint

Key-value stores return the value for the given key.

The key "user2" maps to the value "Bob" in the store.

🧠 Conceptual

advanced

Identify the correct column-family query syntax

Which option shows a valid CQL (Cassandra Query Language) statement to select all columns from the "users" table where "userid" equals 123?

A SELECT * FROM users WHERE userid = 123;

B SELECT ALL FROM users WHERE userid == 123;

C GET * FROM users WHERE userid = 123;

D SELECT * FROM users WHERE userid = '123';

💡 Hint

CQL syntax is similar to SQL but uses single equals for conditions.

Option A uses correct CQL syntax with single equals and semicolon.

🔍 Analysis

expert

Find the error in a graph database query

Given this Cypher query to find friends of 'Alice': MATCH (a:Person {name: 'Alice'})-[:FRIEND]->(friend) RETURN friend.name, which option describes the error if the query returns no results despite data existing?

A The node label should be lowercase :person instead of :Person

B The property name should be friend.names instead of friend.name

C The relationship type should be lowercase :friend instead of :FRIEND

D The query is missing a WHERE clause to filter by name

💡 Hint

Relationship types in Cypher are case-sensitive.

Cypher relationship types are case-sensitive; if data uses :friend but query uses :FRIEND, no matches occur.

Practice

1. Which NoSQL database type stores data as flexible JSON-like documents, allowing nested structures?

easy

A. Document database

B. Key-value database

C. Column database

D. Graph database

Solution

1. Step 1: Understand document database structure

   Document databases store data in JSON-like documents that can have nested fields and arrays.

2. Step 2: Compare with other NoSQL types

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

3. Final Answer:

   Document database -> Option A

4. Quick Check:

   Nested JSON-like data = Document database [OK]

Hint: Documents = JSON-like nested data [OK]

Common Mistakes:

• Confusing key-value stores with document stores
• Thinking column stores store JSON documents
• Mixing graph databases with document databases

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

easy

A. Stores data as interconnected nodes and edges

B. Stores data in tables with rows and columns

C. Stores data as simple pairs of keys and values

D. Stores data as nested JSON documents

Solution

1. Step 1: Define key-value store

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

2. Step 2: Eliminate other options

   Graphs use nodes and edges, column stores use tables with columns, and document stores use nested JSON.

3. Final Answer:

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

4. Quick Check:

   Key-value = key and value pairs [OK]

Hint: Key-value = simple pairs [OK]

Common Mistakes:

• Confusing graph databases with key-value stores
• Thinking key-value stores use tables
• Mixing document stores with key-value stores

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 list of key-value pairs with friend names

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

C. A table with columns for friend names and ages

D. A JSON document with nested friend details

Solution

1. Step 1: Understand graph database query output

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

2. Step 2: Compare with other data formats

   Key-value pairs, tables, and JSON documents are typical of other NoSQL types, not graph databases.

3. Final Answer:

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

4. Quick Check:

   Graph query = nodes and edges [OK]

Hint: Graph = nodes connected by edges [OK]

Common Mistakes:

• Expecting JSON documents from graph queries
• Thinking graph data is tabular
• Confusing key-value pairs with graph edges

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

medium

A. Used SQL SELECT syntax without specifying columns

B. Used key-value pairs instead of column names

C. Tried to traverse edges like in a graph database

D. Tried to query nested JSON documents directly

Solution

1. Step 1: Understand column store query requirements

   Column stores require specifying columns in queries; missing columns causes errors.

2. Step 2: Analyze other options

   Nested JSON is for document stores, key-value pairs are for key-value stores, and edge traversal is for graph databases.

3. Final Answer:

   Used SQL SELECT syntax without specifying columns -> Option A

4. Quick Check:

   Column store queries need columns specified [OK]

Hint: Specify columns in column store queries [OK]

Common Mistakes:

• Using document store syntax on column stores
• Trying graph traversal in column stores
• Confusing key-value pairs with columns

5. You need to design a social network app that quickly finds mutual friends between users. Which NoSQL database type is best suited and why?

hard

A. Column database, because it stores data in columns

B. Key-value store, because it is the fastest for any query

C. Document database, because it stores nested user data

D. Graph database, because it efficiently handles relationships

Solution

1. Step 1: Identify the data need for mutual friends

   Finding mutual friends requires exploring relationships between users efficiently.

2. Step 2: Match data need to NoSQL type

   Graph databases specialize in storing and querying relationships like friendships quickly.

3. Step 3: Evaluate other options

   Document stores handle nested data but not relationship queries efficiently; key-value stores are simple but not optimized for relationships; column stores focus on columnar data.

4. Final Answer:

   Graph database, because it efficiently handles relationships -> Option D

5. Quick Check:

   Relationship queries = Graph database [OK]

Hint: Use graph DB for relationship-heavy apps [OK]

Common Mistakes:

• Choosing document DB for relationship queries
• Assuming key-value stores handle complex queries
• Confusing column stores with relationship handling