SQL (PostgreSQL, MySQL) for most applications — structured data, relationships, complex queries. NoSQL (MongoDB, Redis, DynamoDB) for specific use cases — flexible schemas, massive scale, key-value access.
Side-by-side
| SQL (Relational) | NoSQL (Document/Key-Value) | |
|---|---|---|
| Data model | Tables with rows and columns | Documents (JSON), key-value, graph |
| Schema | Fixed (defined upfront) | Flexible (schema-less) |
| Relationships | Joins (built-in) | Embedded documents or manual |
| Query language | SQL (standardized) | Varies per database |
| Transactions | ACID (strong guarantees) | Varies (often eventual consistency) |
| Scaling | Vertical (bigger server) | Horizontal (more servers) |
| Best for | Most apps, complex queries | Specific use cases (see below) |
When to use SQL
- Most web applications — users, orders, products, posts. Relational data.
- Complex queries — joins, aggregations, reporting.
- Data integrity matters — financial data, inventory, anything where consistency is critical.
- You don’t know your query patterns yet — SQL is flexible enough to handle anything.
Use PostgreSQL. It’s the best general-purpose database in 2026.
When to use NoSQL
- Caching — Redis (key-value, in-memory)
- Flexible/evolving schemas — MongoDB (document store) for rapid prototyping
- Massive write throughput — DynamoDB, Cassandra
- Real-time data — Redis pub/sub, Firebase
- Graph relationships — Neo4j (social networks, recommendation engines)
- Session storage — Redis
The honest take
Start with SQL (PostgreSQL). It handles 95% of use cases. Add NoSQL databases for specific needs:
- Redis for caching and sessions
- MongoDB if you genuinely need flexible schemas
- DynamoDB if you’re on AWS and need massive scale
The “SQL vs. NoSQL” debate is mostly over. The answer is: use SQL as your primary database, and add NoSQL databases for specific use cases where they excel.
Modern PostgreSQL even blurs the line — its JSONB support gives you document-database flexibility with relational guarantees.
See also: SQL cheat sheet | PostgreSQL cheat sheet | MongoDB cheat sheet | Redis cheat sheet