What is the difference between SQL and NoSQL databases?

SQL and NoSQL databases differ primarily in their data models, scalability approaches, and use cases. SQL databases are relational, meaning they organize data into structured tables with predefined schemas, using rows and columns. They rely on SQL (Structured Query Language) for defining and manipulating data, enforcing strict relationships between tables through foreign keys. Examples include MySQL, PostgreSQL, and Oracle. NoSQL databases, on the other hand, are non-relational and support flexible data models like documents, key-value pairs, graphs, or wide-column stores. They don’t require a fixed schema, allowing data to be stored in formats like JSON (e.g., MongoDB) or distributed across clusters (e.g., Cassandra). This makes NoSQL suitable for unstructured or rapidly changing data.

The second key difference lies in scalability and transaction handling. SQL databases typically scale vertically (by upgrading server hardware) and prioritize ACID (Atomicity, Consistency, Isolation, Durability) compliance for reliable transactions, making them ideal for applications like banking systems or inventory management where data integrity is critical. NoSQL databases scale horizontally by distributing data across multiple servers, which suits high-throughput or large-scale applications like social media platforms or IoT sensor networks. They often use eventual consistency (where data updates propagate gradually) instead of strict ACID compliance, trading immediate consistency for availability and partition tolerance, as defined by the CAP theorem. For example, a NoSQL database like Amazon DynamoDB can handle millions of requests per second with low latency.

Finally, flexibility and use cases differ significantly. SQL databases excel in scenarios requiring complex queries, joins, and transactions, such as financial reporting or ERP systems. However, schema changes in SQL systems can be slow, requiring migrations. NoSQL databases allow dynamic schema adjustments, making them better for agile development or applications with evolving data requirements, like content management systems or real-time analytics. For instance, a product catalog with varying attributes across items could store data more efficiently in a document database like MongoDB. Developers should choose based on the project’s needs: SQL for structured data with strict integrity, NoSQL for scalability and flexibility with unstructured data.