NoSQL database types (document, key-value, column, graph) in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Document NoSQL database?

A Document NoSQL database stores data as documents, usually in JSON or similar formats. Each document holds data in key-value pairs and can have different structures. It's like storing many notes where each note can have different information.

beginner

How does a Key-Value NoSQL database work?

A Key-Value NoSQL database stores data as pairs of keys and values. You use the key to quickly find the value. Think of it like a dictionary where you look up a word (key) to get its meaning (value).

intermediate

What is special about Column NoSQL databases?

Column NoSQL databases store data in columns instead of rows. This helps when you want to read or write many values from the same column quickly. Imagine a spreadsheet where you focus on one column at a time.

beginner

Explain Graph NoSQL databases in simple terms.

Graph NoSQL databases store data as nodes (things) and edges (connections). They are great for showing relationships, like friends on social media or routes on a map.

intermediate

Which NoSQL database type is best for storing social network connections?

Graph NoSQL databases are best because they focus on relationships between data, like friends or followers, making it easy to explore connections.

Which NoSQL type stores data as flexible documents?

A Document

B Key-Value

C Column

D Graph

Document databases store data as documents with flexible structures.

In which NoSQL type do you find data stored as key and value pairs?

A Graph

B Document

C Column

D Key-Value

Key-Value databases store data as pairs of keys and values.

Which NoSQL type is optimized for reading many values from the same column?

A Graph

B Column

C Document

D Key-Value

Column databases store data by columns, making column-based queries fast.

Which NoSQL type is best for representing relationships like social networks?

A Column

B Document

C Graph

D Key-Value

Graph databases focus on nodes and edges to represent relationships.

If you want to store user profiles with different fields per user, which NoSQL type fits best?

A Document

B Column

C Key-Value

D Graph

Document databases allow flexible fields per document, ideal for varied user profiles.

Describe the four main types of NoSQL databases and give a simple example for each.

Think about how each type organizes data differently.

Explain why you might choose a Graph NoSQL database over a Document database.

Consider the importance of relationships in your data.

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