如何设计MySQL表结构来支持在线考试系统的考试成绩统计？

SELECT e.name AS exam_name, s.score
FROM scores AS s
JOIN exams AS e ON s.exam_id = e.id
WHERE s.student_id = ;

SELECT AVG(score) AS average_score
FROM scores AS s
WHERE s.exam_id = ;

SELECT st.name AS student_name, s.score
FROM scores AS s
JOIN students AS st ON s.student_id = st.id
WHERE s.exam_id = 
AND s.score < ;

SELECT COUNT(*) AS count,
CASE
  WHEN score >= 90 THEN 'A'
  WHEN score >= 80 THEN 'B'
  WHEN score >= 70 THEN 'C'
  WHEN score >= 60 THEN 'D'
  ELSE 'F'
END AS grade
FROM scores
WHERE exam_id = 
GROUP BY grade;