Joins

Part of the MySQL course

Your users live in one table and their orders live in another, and the only thread connecting an order to the person who placed it is a number in a column. That split is deliberate: it means a user's name is written down exactly once, so changing it changes it everywhere.

A join is how you stitch them back together for the length of a single query. Joins are the single biggest reason SQL feels powerful, and also the place beginners lose the most time, almost always because rows silently vanished or silently multiplied. We will go slowly and use data designed to expose both. ...