What are relational and non-relational databases and how do they differ?

Relational databases (RDBMS) are database management systems that store data in the form of tables with clearly defined relationships between them. They use the structured query language SQL and ensure strict data integrity, transactions, and support for ACID.

Non-relational databases (NoSQL) are more flexible systems that do not use tables and relationships in the traditional sense. They can store data as documents (JSON, BSON), key-value pairs, graphs, or columns. NoSQL databases are often used for large volumes of data, high scalability, and flexible schemas.

Main differences:

• Data structure: Relational — tables with a fixed schema; Non-relational — flexible schema or no schema.
• Query language: Relational — SQL; Non-relational — various APIs or query languages.
• Scalability: Relational — more often vertically scalable; NoSQL — horizontally.
• Data integrity: Relational databases provide strict transactions; NoSQL often sacrifices this for performance.

Examples:

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