JOINs

Real data lives across multiple related tables. A JOIN combines rows from two tables based on a matching column.

SELECT users.name, enrollments.purchased_at
FROM users
JOIN enrollments ON users.id = enrollments.user_id;

An INNER JOIN (what plain JOIN means) only returns rows that match in both tables. A LEFT JOIN keeps every row from the left table even if there's no match on the right — useful for "show me every user, including ones with zero enrollments":

SELECT users.name, COUNT(enrollments.id) AS course_count
FROM users
LEFT JOIN enrollments ON users.id = enrollments.user_id
GROUP BY users.name;