LEFT JOIN

LEFT JOIN keeps every row from the left table, even with no match. Missing values become NULL.

SELECT s.name, COUNT(e.course_id) AS courses
FROM students s
LEFT JOIN enrolments e ON e.student_id = s.id
GROUP BY s.name;

name  | courses
------+--------
Ali   | 1
Sara  | 2
Bilal | 0

Bilal has no enrolments, but still appears thanks to LEFT JOIN.