[英]select the most recent answer for each entry MySQL
我覺得這真的很容易做到,我只是在某個地方犯了一些小錯誤。 可能應該補充一點,我是一名老師,而不是編碼員,所以我不太熟悉SQL。 另外,我確實在這里看了一堆問題,但都沒有解決。
我有表格student_answers(id, student_id, question_id, answer, result, date_time)
我想獲取每個學生輸入的最后一個答案的question_id
, answer
, result
和date_time
。 因此,如果他們對問題7回答了3次,我只想查看他們輸入的最后一個answer
和result
。
出於教學目的,我不能簡單地在每行重新輸入答案時進行更新。
我嘗試了以下查詢
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
但這根本不包括多個答案的問題,只有我505學生回答一次的問題。 學生505回答了問題3和4兩次,其余的僅回答了一次,我只看到了1、2和5的結果。
我試過這個查詢
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
但這只給了我3和4,而他只給了我一次。 任何幫助將不勝感激!
這是數據示例:
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
當我運行查詢時,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
通知條目7135和7137被省略,因為每個問題都有一個稍后的答案
檢查以下查詢:(為所有學生提供所有問題的最新答案)
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;
對於特定的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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