How do you choose between SQL and NoSQL databases?

The useful comparison is relational versus non-relational modelling. Relational engines assume normalized tables, joins, and strong constraints. Document, key-value, wide-column, and graph stores optimize for particular shapes and access paths.

SELECT o.id, o.total_cents, c.email
FROM orders o
JOIN customers c ON c.id = o.customer_id
WHERE c.region = 'EU'
  AND o.created_at >= now() - interval '30 days';

Where each model fits

• Relational: financial records, inventory, and bookings needing multi-row transactions
• Document: aggregates read and written as a unit, such as a product catalogue
• Key-value: caching, sessions, rate limits, and high-throughput key lookup
• Graph: relationship-heavy traversal such as social connections or fraud rings

NoSQL is not automatically faster or more scalable. A document store often duplicates data to avoid joins, which becomes an application consistency problem. Relational databases scale reads with replicas and writes with partitioning, but cross-shard transactions add complexity.