How do relational databases differ from non-relational ones? What does the term NoSQL mean?

Relational databases (RDBMS) store data in tables with a fixed schema, where data is linked through keys (e.g., primary and foreign keys). They support SQL for queries and ensure transactional properties (ACID).

Non-relational databases (NoSQL) do not use tables with a fixed schema. They can store data as documents (JSON), key-value pairs, graphs, or columns. NoSQL databases are more flexible in data structure and scale horizontally.

The term NoSQL means "Not Only SQL" — databases that are not limited to the relational model and SQL, offering alternative ways to store and process data, often for large distributed systems.

Examples:

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