SET @courses_completed = '2';
SELECT c.course_id
FROM course c
LEFT JOIN course_prerequisites p ON p.course_id = c.course_id
WHERE NOT FIND_IN_SET(c.course_id, @courses_completed)
GROUP BY c.course_id
HAVING SUM(COALESCE(FIND_IN_SET(p.prerequisite_course_id, @courses_completed), 0) > 0) = COUNT(p.prerequisite_course_id);