The SQL vs NoSQL debate peaked around 2013 when MongoDB was the modern choice and SQL was considered legacy. By 2026, the conversation has shifted: PostgreSQL (with JSONB, full-text search, and extensions) handles the majority of use cases that drove developers to NoSQL, and the strongest case for purpose-built NoSQL databases is narrower than it was a decade ago.
SQL databases (PostgreSQL, MySQL, SQLite) are the right choice for: structured data with known relationships, applications where data integrity and ACID transactions matter, workloads that need complex queries across related entities (joins, aggregations, window functions), and any team with SQL skills. The relational model is not a limitation — it's a tool for expressing constraints that the database enforces for you.
Document databases (MongoDB, Firestore, CouchDB) are the right choice when: your data genuinely has unpredictable or highly variable structure that would require hundreds of nullable columns in a relational table, you need to store deeply nested hierarchical data that's always accessed as a unit (not queried across its sub-properties), or you're building a mobile application where Firestore's client SDK and real-time sync are architectural requirements. The 2026 caveat: PostgreSQL's JSONB handles most of these cases. You can store, index, and query nested JSON in PostgreSQL with the same flexibility as MongoDB.
Key-value stores (Redis, Valkey, DynamoDB) are the right choice for: caching (fast reads by a known key), session storage, rate limiting counters, leaderboards, and any pattern where you look up data by exact key rather than query it by attributes. Key-value is not a general database — it's a specialized tool for specific access patterns.
Wide-column stores (Cassandra, ScyllaDB, DynamoDB in table mode) are the right choice for: massive scale writes (billions of events), time-series data with append-only access patterns, and globally distributed low-latency reads without complex queries. The operational complexity is high.
The practical default for new applications in 2026: start with PostgreSQL. Use Redis/Valkey for caching alongside it. Add a specialized NoSQL store only when PostgreSQL demonstrably fails to meet a specific requirement (write throughput, global distribution, schema flexibility at scale). This avoids the common mistake of choosing NoSQL for flexibility and discovering 18 months later that you need joins.