Question

我知道我必须在这里遗漏一些简单的东西......但是我在编写SQL子查询时遇到了问题。鉴于以下数据

user_id question_id answer_text
89          1           value1
89          2           value2
80          2           value2
99          2           value2
96          1           value1
96          2           value2
111        1           value1
111        2           value2

我需要获取具有两个问题的user_id问题1 ='value1'并且有问题2 ='value2'

以上结果是使用此查询生成的：

SELECT `User_answer`.`user_id`, `User_answer`.`question_id`, `User_answer`.`answer_text` FROM `user_answers` AS `User_answer` WHERE `User_answer`.`question_id` IN (1, 2) AND `User_answer`.`answer_text` IN ('value1', 'value2')

但当然这会使回答问题1的用户返回“value1”但问题2的回答与“value2”完全不同（反之亦然）。

我需要能够添加更多条件，但我想我可以补充说，如果我至少可以使这两个工作。

Answer 1

一种方法是使用自联接，例如

select a1.user_id, a1.answer_text AS Answer1, a2.answer_text AS Answer2
from user_answers a1 join user_answers a2 on a1.userid=a2.userid and a1.question_id=1     and a2.question_id=2
where a1.answer_text <> a2.answer_text

对不起，如果语法稍微偏离（我使用MySQL已经有一段时间了），但你明白了。

Answer 2

我会使用这样的内连接来执行此操作：

SELECT
  user_id
FROM
  user_answers AS a1
  INNER JOIN user_answers AS a2 ON
    a2.user_id = a1.user_id
    AND a2.question_id=2
    AND a2.answer_text='value2'
WHERE
  a1.question_id=1
  AND a1.answer_text='value1'

转换为：

查找所有答案，其中question_id为1，answer_text为'value1'
对于这些答案，请找到具有相同用户ID和question_id为2以及answer_text为“value2”的相应答案。
做一个内连接，即扔掉那些不满足两者的连接。

Answer 3

我尝试在没有子查询的情况下这样做：

SELECT `User_answer`.`user_id`, 
  MAX(CASE `User_answer`.`question_id` WHEN 1 THEN `User_answer`.`answer_text` END) AS `q1_answer`,
  MAX(CASE `User_answer`.`question_id` WHEN 2 THEN `User_answer`.`answer_text` END) AS `q2_answer`
FROM `user_answers` AS `User_answer` 
WHERE (`User_answer`.`question_id` = 1 AND `User_answer`.`answer_text` = 'value1')
  OR  (`User_answer`.`question_id` = 2 AND `User_answer`.`answer_text` = 'value2')
GROUP BY `User_answer`.`user_id`
HAVING COUNT(DISTINCT `User_answer`.`question_id`) = 2;

编写SQL子查询的问题

3 个答案: