Untitled

Mysql: 2 table Join
SELECT sc.* FROM student_courses as sc
LEFT JOIN student_questionnaires as sq on sq.course_id=sc.course_id AND sq.student_id=sc.student_id

SELECT sc.*
FROM
  student_courses sc
  LEFT JOIN student_questionnaires sq ON sc.student_id = sq.student_id AND sc.course_id = sq.course_id
WHERE sq.course_id IS NULL

SELECT sc.*
FROM student_courses as sc
WHERE sc.student_id NOT IN (SELECT student_id FROM student_questionnaires)

SELECT DISTINCT Student_ID
FROM Student_Courses
WHERE LENGTH(COALESCE(Course_ID)) <> 0 AND Student_ID NOT IN
    (SELECT student_id Student_Questionnaires)