What is the difference between relational and non-relational databases?

Relational databases (RDBMS) organize data into tables with well-defined schemas, where data is linked through keys (primary and foreign). They use SQL for queries and ensure transactional integrity, data consistency, and support ACID properties.

Non-relational databases (NoSQL) do not use tables and schemas in the traditional sense. They can store data as documents (JSON, BSON), key-value pairs, graphs, or columns. NoSQL databases are more flexible in data structure, scale horizontally better, and are suitable for large volumes of unstructured data, but often sacrifice strict consistency for performance and scalability.

Examples:

• Relational DB: PostgreSQL, MySQL
• Non-relational DB: MongoDB (documents), Redis (key-value), Neo4j (graphs)