Table 的設計要以「需求」為出發點考量
就是「要查詢那些資訊」？例如：
1.學生問了那些問題
2.學生回答了那些問題
3.問題有那些答案

根據你的文字描述製圖如下

坦白說我看不太懂,例如：
1.student_id 和 question_id 和 answer_id 的關係是什麼？
2.會不會問題還沒有答案的情形？
3.一個問題由一個學生發問；答案需要記錄是那個學生回答的嗎？

我稍微改了一下如下

如果上圖可以滿足題意的所有需求
再來討論細節
不然就算我胡說八道
不用理我

另外提供今天才發現的好物
就是免費免註冊線上畫 ERD的服務
下面是產生上面兩張圖的 source

//// -- LEVEL 1
//// -- Tables and References

// Creating tables
Table student as s {
  student_id int [pk] // auto-increment
}

Table question as q {
  question_id int [pk]
 }

Table answer as a {
  answer_id int [pk]
  question_id int
 }

Table student_qa as qa {
  student_id int [pk]
  question_id int [pk]
  answer_id int [pk]
 }

Ref: "student_qa"."answer_id" < "answer"."answer_id"

//// -- LEVEL 1
//// -- Tables and References

// Creating tables
Table student as s {
  student_id int [pk, increment] // auto-increment
}

Table question as q {
  question_id int [pk]
  student_id int 
 }

Table answer as a {
  answer_id int [pk]
  student_id int
 }

//// -- LEVEL 2

Table question_answer as qa {
  question_id int [pk]
  answer_id int [pk]
 }



// Creating references
// You can also define relaionship separately
// > many-to-one; < one-to-many; - one-to-one
Ref: s.student_id < q.student_id
Ref: s.student_id < a.student_id

Ref: qa.question_id - q.question_id
Ref: qa.answer_id - a.answer_id