When should you choose a Relational SQL database versus a NoSQL Document/Key-Value store?
❓ Interview Prompt: Compare SQL (Relational, ACID transactions, complex JOINs) vs NoSQL (Document/Key-Value, horizontal scaling, flexible schemas).
Evaluating database paradigms:
| Criteria | Relational SQL (Postgres, MySQL) | NoSQL (MongoDB, DynamoDB, Redis) |
|---|---|---|
| Data Structure | Structured tables with fixed schemas & Foreign Keys | Unstructured / JSON Documents / Key-Value pairs |
| ACID & Consistency | Strict ACID transactions & strong consistency | Eventual consistency; high write throughput |
| Scaling | Vertical scaling (Scale up); Read Replicas | Horizontal scaling (Scale out sharding out of box) |
| Best For | Financial transactions, e-commerce orders, relational data | High-velocity logs, user session caches, real-time feeds |
Code Implementation Example
{
"selectedDatabase": "Relational SQL (PostgreSQL)",
"rationale": "Requires multi-table ACID transactions and strict schema validation for user billing balances"
}Choose Relational SQL for complex ACID financial transactions, and NoSQL for high-throughput, horizontally scalable unstructured data.