为每个条目选择最近的答案MySQL

Question

我觉得这应该很容易做，我只是在某个地方犯了一些小错误。大概应该补充说我是老师而不是编码员，所以我不熟悉SQL。另外，我在这里看到了一堆问题，但都没有工作。

我有表student_answers(id, student_id, question_id, answer, result, date_time)我想获得一个学生对每个问题输入的最后答案question_id，answer，result和date_time。所以如果他们回答三次问题7，我只想看到他们输入的最后answer和result。

出于教学目的，我不能简单地更新每一行，因为他们重新输入答案。

我尝试以下查询

SELECT id, question_id, answer, result, date FROM Student_Answers 
WHERE student_id = 505 AND question_id in (select id from Test_Questions q where q.test_id = 37) 
Group by question_id 
having date = max(date) 
ORDER BY Student_Answers`.`question_id` ASC 

但不包括与多个答案的问题在所有，只有我，学生505回答一次问题。学生505回答问题3和4两次，剩下的只有一次，我只看到了结果1，2和5

我尝试这个查询

SELECT 
    b.* 
FROM 
    (
     SELECT question_id, MAX(date) AS maxdate 
     FROM TP_Student_Answers 
     GROUP BY question_id 
    ) a 
INNER JOIN 
    TP_Student_Answers b ON 
     a.question_id = b.question_id AND 
     a.maxdate = b.date 
     and b.student_id = 505 
     and b.question_id in (select id from TP_Questions q where q.test_id = 37) 
ORDER BY 
    b.question_id 

但是这对只给了我3和4没有一次他只尝试过一次。任何帮助将不胜感激！

这是数据的一个样本：

id student_id question_id answer result  date 

7133  505 1  a correct 2012-11-16 09:03:58 

7134 505 2  c wrong 2012-11-16 09:03:58 

7135 505 3  e wrong 2012-11-16 09:03:58 

7136 505 3  d wrong 2013-12-16 09:03:58 

7137 505 4  c correct 2012-11-16 09:03:58 

7138 505 4  d wrong 2013-12-16 09:03:58 

7139 505 5  blank 2012-11-16 09:03:58 

当我运行查询我想看看：

7133  505 1  a correct 2012-11-16 09:03:58 

7134 505 2  c wrong 2012-11-16 09:03:58 

7136 505 3  d wrong 2013-12-16 09:03:58 

7138 505 4  d wrong 2013-12-16 09:03:58 

7139 505 5  blank 2012-11-16 09:03:58 

通知条目7135和7137被省略，因为还有更晚的答案对于其中的每一个问题

Answer 1

选择此查询:(给所有学生提供所有问题的最新答案）

SELECT id, student_id, question_id, answer, result, date_time 
    FROM (SELECT *, 
       CASE 
        WHEN (@prevQ = question_id AND @prevS = student_id) 
        THEN 
        @marker := 0 
        ELSE 
        @marker := 1 
       END 
        AS marker, 
       @prevS := student_id, 
       @prevQ := question_id 
        FROM student_answers 
       ORDER BY student_id ASC, question_id ASC, date_time DESC) aView, 
       (SELECT @prevQ = -1, @prevS = -1) a 
WHERE marker = 1; 

和特定student_id和具体question_id S：

SELECT id, student_id, question_id, answer, result, date_time 
    FROM (SELECT *, 
       CASE 
        WHEN (@prevQ = question_id AND @prevS = student_id) 
        THEN 
        @marker := 0 
        ELSE 
        @marker := 1 
       END 
        AS marker, 
       @prevS := student_id, 
       @prevQ := question_id 
      FROM student_answers 
     WHERE  student_id = 501 
       AND question_id IN (SELECT id 
            FROM TP_Questions q 
            WHERE q.test_id = 37) 
     ORDER BY student_id ASC, question_id ASC, date_time DESC) aView, 
     (SELECT @prevQ = -1, @prevS = -1) a 
WHERE marker = 1;