What are the differences between relational and non-relational databases in terms of structure and usage?

Relational databases (RDBs) are built on tables with clearly defined schemas (structures), where data is linked through keys (for example, primary and foreign keys). They use SQL language for queries and are well-suited for complex transactions and strict data integrity.

Non-relational databases (NoSQL) do not require a fixed schema and can store data in various formats: documents (JSON, BSON), key-value, graphs, column stores. They are more flexible, scale horizontally, and are often used for large volumes of data with less strict structural requirements.

Example:

• Relational DB: a user table with columns id, name, email.
• Non-relational DB: a collection of documents, where each document is a JSON with arbitrary user fields.