What is the difference between SQL and NoSQL databases

Introduction

SQL databases use structured schemas and relational tables. NoSQL databases use flexible schemas and various data models including documents, key-value pairs, wide columns, and graphs.

Schema Flexibility

SQL enforces rigid schemas with typed columns and relationships. NoSQL allows fields to vary between records, making it easier to evolve data models without migrations.

Scalability

SQL databases traditionally scale vertically by adding CPU and RAM to a single server. NoSQL databases are designed for horizontal scaling across commodity hardware, though modern SQL databases also support sharding.

Consistency Models

SQL databases typically provide strong ACID consistency. NoSQL often prioritizes availability and partition tolerance (CAP theorem), offering eventual consistency. If transactional safety is critical, see what are database transactions and when to use them.

Choosing Between Them

Use SQL when you need complex joins, strong consistency, and structured reporting. Use NoSQL for rapid prototyping, massive scale, or unstructured data. Many systems use both in a polyglot persistence architecture. For schema design in SQL, refer to how to design a database schema from scratch.