Решение

SELECT
  s.student_id,
  s.student_name,
  sub.subject_name,
  COUNT(e.student_id) AS attended_exams
FROM Students AS s
CROSS JOIN Subjects AS sub
LEFT JOIN Examinations AS e
  ON s.student_id = e.student_id
  AND sub.subject_name = e.subject_name
GROUP BY
  s.student_id,
  s.student_name,
  sub.subject_name
ORDER BY
  s.student_id,
  sub.subject_name;