Types of Databases with Examples

Types of Databases:

Relational Databases
Non-Relational Databases (NoSQL)
Column Databases
Graph Databases
Key-Value Databases

1. Relational Databases:

A database that stores data in tables with rows and columns, where tables are connected using specific columns (keys).

Example: MySQL, PostgreSQL

Data Format: Tabular (rows and columns)

Example:

Table: Employees
| ID  | Name      | Department | Salary  | Hire_Date  |
|-----|-----------|------------|---------|------------|
| 1   | Alice     | HR         | 50000   | 2023-01-15 |
| 2   | Bob       | IT         | 60000   | 2022-06-10 |
| 3   | Charlie   | Sales      | 55000   | 2024-03-01 |

Table: Departments
| Dept_ID | Dept_Name | Location   |
|---------|-----------|------------|
| 1       | HR        | Building A |
| 2       | IT        | Building B |
| 3       | Sales     | Building C |

2. Non-Relational Databases (NoSQL):

A database that doesn’t use tables; it’s flexible and can store data in various ways like documents, key-value pairs, or graphs.

Example: MongoDB, Cassandra

Data Format: Varies (e.g., JSON-like documents)

Example:

Document 1:
{
  "employee_id": 1,
  "name": "Alice",
  "department": "HR",
  "skills": ["communication", "recruiting"],
  "salary": 50000,
  "hire_date": "2023-01-15"
}

Document 2:
{
  "employee_id": 2,
  "name": "Bob",
  "department": "IT",
  "skills": ["coding", "networking"],
  "salary": 60000,
  "hire_date": "2022-06-10"
}

3. Column Databases:

A database that stores data by columns instead of rows, optimized for fast retrieval of specific columns.

Example: HBase, Google Bigtable

Data Format: Column-based storage

Example:

Column Family: EmployeeData
| Row Key | Name    | Department | Salary | Hire_Date  |
|---------|---------|------------|--------|------------|
| emp1    | Alice   | HR         | 50000  | 2023-01-15 |
| emp2    | Bob     | IT         | 60000  | 2022-06-10 |
| emp3    | Charlie | Sales      | 55000  | 2024-03-01 |



(Stored as:
 - Name column: "Alice, Bob, Charlie"
 - Department column: "HR, IT, Sales"
 - Salary column: "50000, 60000, 55000"
 - Hire_Date column: "2023-01-15, 2022-06-10, 2024-03-01")

4. Graph Databases

A database that stores data as nodes (entities) and edges (relationships), great for showing connections.

Example: Neo4j, ArangoDB

Data Format: Nodes and edges

Example:

(Alice) --[WORKS_IN]--> (HR_Dept)
(Bob)   --[WORKS_IN]--> (IT_Dept)
(Alice) --[FRIENDS_WITH]--> (Bob)

Node 1: { "name": "Alice", "role": "Recruiter", "salary": 50000 }
Node 2: { "name": "Bob", "role": "Developer", "salary": 60000 }
Node 3: { "dept_name": "HR_Dept", "location": "Building A" }
Node 4: { "dept_name": "IT_Dept", "location": "Building B" }
Edge 1: { "relationship": "WORKS_IN", "since": "2023-01-15" }
Edge 2: { "relationship": "FRIENDS_WITH", "years": 2 }

5. Key-Value Databases:

A simple database that stores data as a collection of key-value pairs, like a dictionary.

Example: Redis, DynamoDB

Data Format: Key-value pairs

Example:

Key: "employee:1"  -> Value: "name:Alice, dept:HR, salary:50000, hire_date:2023-01-15"
Key: "employee:2"  -> Value: "name:Bob, dept:IT, salary:60000, hire_date:2022-06-10"
Key: "employee:3"  -> Value: "name:Charlie, dept:Sales, salary:55000, hire_date:2024-03-01"
Key: "dept:HR"     -> Value: "location:Building A, head:Alice"
Key: "dept:IT"     -> Value: "location:Building B, head:Bob"

12+ Interview Questions Repeatedly Asked About Databases

Question 1. What is a database?

Answer: A database is a system to store and manage data in an organized way. It makes finding and using information easy and efficient.

Example: A company storing employee names, IDs, and salaries.

Question 2. What’s the difference between relational and non-relational databases?

Answer: Relational databases use tables with rows and columns to store data with a fixed structure. Non-relational databases are more flexible, using things like documents or pairs instead of tables.

Example: Relational: table of users | Non-relational: {"name": "Alice", "age": 25}.

Question 3. What is a primary key?

Answer: A primary key is a unique value that identifies each row in a table. It ensures no two rows are the same.

Example: "1" for Alice and "2" for Bob in an employee list.

Question 4. What is a foreign key?

Answer: A foreign key is a column in one table that links to the primary key in another table. It connects related data across tables.

Example: Order table with Customer ID linking to a customers table.

Question 5. What is a NoSQL database?

Answer: A NoSQL database doesn’t use tables and can handle flexible or large amounts of data. It’s great for apps needing speed or changing data types.

Example: {"user": "Alice", "hobbies": ["gaming"]} in MongoDB.

Question 6. Why use a column database?

Answer: A column database stores data by columns, not rows, making it fast for analyzing specific data points. It’s useful for big reports or stats.

Example: Quickly grabbing all salaries: 50000, 60000.

Question 7. What is a graph database used for?

Answer: A graph database stores data as nodes and connections, perfect for showing relationships. It’s used in things like social networks or maps.

Example: Alice linked to Bob as friends.

Question 8. What is a key-value database?

Answer: A key-value database stores data as simple pairs, where a key points to a value. It’s fast and easy for basic lookups or caching.

Example: "user:1" points to "Alice, 25".

Question 9. What does normalization mean?

Answer: Normalization splits data into multiple tables to remove repeats and keep it clean. It helps avoid errors and saves space.

Example: Separating employee names and department details.

Question 10. How are relational and NoSQL databases different for data structure?

Answer: Relational databases use fixed tables with a set structure for organized data. NoSQL databases use flexible formats like documents for less rigid data.

Example: Table of users vs. {"name": "Bob", "age": 30}.

Question 11. When would you use a relational database?

Answer: Use a relational database when data is structured and has clear relationships, like in finance or inventory. It keeps everything connected and consistent.

Example: Tables for bank accounts and transactions.

Question 12. When would you pick a graph database over a key-value one?

Answer: Pick a graph database when you need to show complex relationships between things. Use a key-value database for simple, fast data lookups instead.

Example: Graph: Alice buys a product | Key-value: "session:123" = "Alice".

Types of Databases with Examples