Question

我有一个案例，我需要选择在sql中回答该问题的每个问题用户数我的表看起来像

table 1: questionnaire_question  
id  - question_text - thecorrectAnswer

table 2 :questionnaire_answers  
id -  user_id -  question_id -  user_answer - correct

正确列的值为0或1表示用户答案是否正确

Answer 1

您可以加入表格并count case表达式来检查正确性：

SELECT q.id, cnt
FROM   questionnaire_question q
JOIN   (SELECT question_id, COUNT(CASE correct WHEN 1 THEN 1 END) AS cnt
        FROM   questionnaire_answers
        GROUP BY question_id) a ON q.id = a.question_id

Answer 2

使用LEFT JOIN，GROUP BY和COUNT

select
  q.id,
  count(distinct a.user_id) cnt  -- can be replaced with count(*) 
                                 -- if a user has only at max one correct answer
                                 -- for a given question
from questionnaire_question q
left join questionnaire_answers a
on q.id = a.question_id
where correct = 1
group by q.id;

Answer 3

如果问题没有正确答案，则使用子查询，它将返回0

试试这个：

<div style="background-color:red;width:100vw;height:100vh">
</div>
<div>
    hi
 <ul>
  <li>ho</li>
  <li>ho</li>
  <li>ho</li>
  <li>ho</li>
  <li>ho</li>
  <li>ho</li>
 </ul>
</div>

Answer 4

select question_id, count(user_id)
from questionnaire_answers
where correct = 1
group by question_id

不需要加入，因为您已经通过包含正确的列来对模式进行非规范化。

得到每个问题，正确回答的用户数量

4 个答案: