A JOIN connects two tables

Data is often split across tables: students in one, classes in another. Each student row stores a class_id, and each class row has an id. JOIN lets one query use both tables, and ON says which two columns to compare — here class_id and id. Where the values are the same, the student and the class are shown together on one row.

SELECT s.name, c.title
FROM students s
JOIN classes c ON c.id = s.class_id;