[英]select the most recent answer for each entry MySQL
我觉得这真的很容易做到,我只是在某个地方犯了一些小错误。 可能应该补充一点,我是一名老师,而不是编码员,所以我不太熟悉SQL。 另外,我确实在这里看了一堆问题,但都没有解决。
我有表格student_answers(id, student_id, question_id, answer, result, date_time)
我想获取每个学生输入的最后一个答案的question_id
, answer
, result
和date_time
。 因此,如果他们对问题7回答了3次,我只想查看他们输入的最后一个answer
和result
。
出于教学目的,我不能简单地在每行重新输入答案时进行更新。
我尝试了以下查询
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
但这根本不包括多个答案的问题,只有我505学生回答一次的问题。 学生505回答了问题3和4两次,其余的仅回答了一次,我只看到了1、2和5的结果。
我试过这个查询
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
但这只给了我3和4,而他只给了我一次。 任何帮助将不胜感激!
这是数据示例:
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
当我运行查询时,我希望看到:
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,因为每个问题都有一个稍后的答案
检查以下查询:(为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;
对于特定的student_id
和特定的question_id
:
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;
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