SQL vs NoSQL — When to Use Which

Choosing the right database is one of the most critical architectural decisions.

SQL Databases

Examples: PostgreSQL, MySQL, SQLite

Strengths:

• ACID transactions
• Complex joins and queries
• Strong schema enforcement
• Mature ecosystem

Weaknesses:

• Harder to scale horizontally
• Schema changes are expensive

NoSQL Databases

Key-Value

Examples: Redis, DynamoDB Use case: Session storage, caching

Document

Examples: MongoDB, CouchDB Use case: Product catalogs, user profiles

Column-Family

Examples: Cassandra, HBase Use case: Time-series, write-heavy workloads

Graph

Examples: Neo4j, Amazon Neptune Use case: Social networks, recommendation engines

Decision Matrix

Need ACID transactions?  → SQL
High write throughput?   → Cassandra
Flexible schema?         → MongoDB
Fast key lookups?        → Redis
Relationship queries?    → Neo4j

Polyglot Persistence

Modern systems use multiple databases:

• PostgreSQL for orders/transactions
• Redis for session cache
• Elasticsearch for search
• Cassandra for activity feeds

Conclusion

There is no single best database. Understand your access patterns, consistency requirements, and scale before choosing.