Backend Fundamentals: 02

1. SQL vs NOSQL (When to Use What)

What is SQL?

SQL (Structured Query Language) databases are relational databases that store data in structured tables (rows and columns) with predefined schemas.

Examples

• MySQL
• PostgreSQL
• Microsoft SQL Server

Key Characteristics

• Structured schema (schema-on-write)
• Supports JOIN operations
• Strong ACID compliance
• Best for structured and relational data

What is NoSQL?

NoSQL databases are non-relational databases designed for flexibility, scalability, and handling large volumes of distributed data.

Types

• Document (e.g., MongoDB)
• Key-Value (e.g., Redis)
• Column-Family (e.g., Cassandra)
• Managed NoSQL (e.g., DynamoDB)

Key Characteristics

• Flexible schema (schema-on-read)
• Horizontally scalable
• Optimized for large-scale distributed systems
• Often follow BASE consistency model

Core Differences

| Feature        | SQL                        | NoSQL                           |
|----------------|----------------------------|---------------------------------|
| Data Model     | Tables (Relational)        | Document / Key-Value / Column   |
| Schema         | Fixed, predefined          | Dynamic, flexible               |
| Scaling        | Vertical (scale-up)        | Horizontal (scale-out)          |
| Transactions   | Strong ACID                | BASE / Eventual consistency     |
| Relationships  | Strong (JOIN support)      | Typically denormalized          |

When to Use SQL

Use SQL when:

• You need complex queries and joins
• Strong data integrity is required
• The system is transactional
• Data structure is stable and well-defined

Examples

• Banking systems
• ERP systems
• E-commerce order management

When to Use NoSQL

Use NoSQL when:

• You need high scalability
• Data structure changes frequently
• Large volume of semi-structured/unstructured data
• Real-time or distributed systems

Examples

• Social media feeds
• Logging systems
• IoT platforms
• Caching layers