MySQL：通過查詢獲取組中的最新值

Question

我有兩個表questions和answered。 answered包含所有用戶的所有問題的答案。一位用戶可以多次回答問題。一個問題可以被正確或不正確地回答。

我在查找一個查詢，它將返回一個類別中所有問題的正確答案和錯誤答案的計數。 儘管我只想使用最新的答案。因此，如果用戶之前正確回答了相同的問題，並且正確地更近一些，我只想計算最新的 - 正確的 - 。

這是我走到這一步：

http://sqlfiddle.com/#!2/31e2e/2/0

SELECT a.correct, count(*) as count 
FROM answered a JOIN questions q ON a.question_id = q.id 
WHERE a.user_id = 1 AND q.category_id = 1 
GROUP BY correct 

它返回

| CORRECT | COUNT | 
----------------------- 
| 0  | 2  | 
----------------------- 
| 1  | 4  | 
----------------------- 

我要的是

| CORRECT | COUNT | 
----------------------- 
| 0  | 1  | 
----------------------- 
| 1  | 2  | 
----------------------- 

Answer 1

下面是該查詢你需要：

SELECT a.correct, count(*) as counter 
FROM answered a 
JOIN (SELECT user_id, question_id, max(created) as maxCreated 
     FROM answered 
     GROUP BY user_id, question_id) aux 
    ON a.user_id = aux.user_id AND 
    a.question_id = aux.question_id AND 
    a.created = aux.maxCreated 
JOIN questions q ON a.question_id = q.id 
WHERE a.user_id = 1 AND q.category_id = 1 
GROUP BY a.correct 

使用aux子查詢與最後答案從給定用戶問題只選擇行。

Answer 2

如果你需要計算的唯一正確的答案時，他們會給出：

select b.correct, count(*) from 
(
SELECT a.question_id, max(a.correct) correct 
FROM answered a JOIN questions q ON a.question_id = q.id 
WHERE a.user_id = 1 AND q.category_id = 1 
GROUP BY a.question_id 
) B 
group by b.correct 

如果以後不正確的答案消除正確的，你只需要最後的/當前的結果：

select a.correct,count(*) from 
(
select user_id, question_id, max(created) maxDate 
from answered a WHERE a.user_id = 1 
group by a.user_id, a.question_id 
) m 

join answered a 
on m.question_id=a.question_id 
    and m.user_id=a.user_id 
    and m.maxDate=a.created 
join questions q on a.question_id=q.id 

where q.category_id = 1 

group by a.correct