What is a relational database and what does a basic SELECT do?

A relational database stores data in tables with defined columns and types. Relationships connect rows in different tables, usually using keys. SQL is a declarative language: you describe what you want, and the engine plans how to fetch it. A SELECT reads rows, often with WHERE filters and JOINs to combine data. The backend still enforces who may run which query, usually through an application user or connection pool, not by exposing raw SQL to random clients.

SELECT u.email, o.total
FROM users AS u
JOIN orders AS o ON o.user_id = u.id
WHERE u.id = 42;

Start simple: try this concept in a tiny project before moving to advanced tools.