选择在MySQL中将表的行作为列获取的查询
我有一个MySQL数据库,有两个问答表。每个问题有一个正确答案和三个错误答案。每个问题总是有四个答案,只有一个是正确的 各表如下:选择在MySQL中将表的行作为列获取的查询,mysql,database,select,Mysql,Database,Select,我有一个MySQL数据库,有两个问答表。每个问题有一个正确答案和三个错误答案。每个问题总是有四个答案,只有一个是正确的 各表如下: CREATE TABLE `question` ( `id_question` smallint(5) unsigned NOT NULL auto_increment, `text` varchar(255) collate utf8_unicode_ci default NULL, PRIMARY KEY (`id_question`), ) EN
CREATE TABLE `question` (
`id_question` smallint(5) unsigned NOT NULL auto_increment,
`text` varchar(255) collate utf8_unicode_ci default NULL,
PRIMARY KEY (`id_question`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
CREATE TABLE `answer` (
`id_answer` mediumint(8) unsigned NOT NULL auto_increment,
`id_question` smallint(5) unsigned NOT NULL,
`is_correct` tinyint(1) NOT NULL,
`text` varchar(45) collate utf8_unicode_ci default NULL,
PRIMARY KEY (`id_answer`,`id_question`),
KEY `fk_id_question_idx` (`id_question`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
我需要一个select查询的帮助。我想得到一个表格,行中有问题,列中有四个答案(首先是正确的答案,然后是其他三个)。到目前为止,我能够得到如下输出:
Question | Answer | Is_Correct
-------------------------------
Question 1 Answer 1-1 1
Question 1 Answer 1-2 0
Question 1 Answer 1-3 0
Question 1 Answer 1-4 0
Question 2 Answer 2-1 1
Question 2 Answer 2-2 0
Question 2 Answer 2-3 0
Question 2 Answer 2-4 0
...
我怎样才能得到以下结果
Question | Correct_Answer | Incorrect_answer1 | Incorrect_answer2 | Incorrect_answer3
--------------------------------------------------------------
Question 1 Answer 1-1 Answer 1-2 Answer 1-3 Answer 1-4
Question 2 Answer 2-1 Answer 2-2 Answer 2-3 Answer 2-4
为此构建一个动态透视查询需要大量的工作。相反,我可能要做的是使用MySQL的聚合函数创建一个逗号分隔的
不正确答案
字段列表,同时将正确答案
分隔为自己的列:
SELECT
question.`text`,
/* Separate the one where Is_Correct = 1 and discard the others with a MAX() aggregate */
MAX(CASE WHEN Is_Correct = 1 THEN answer.`text` ELSE NULL END) AS Correct_Answer,
/* Group the rows where Is_Correct = 0 into a comma-separated list */
GROUP_CONCAT(CASE WHEN Is_Correct = 0 THEN answer.`text` ELSE NULL END) AS Incorrect_answers
FROM
question
JOIN answer ON question.id_question = answer.id_question
GROUP BY Question.`text`
应用程序代码接收到的结果如下所示:
Question Correct_Answer Incorrect_answers
--------------------------------------------------------------
Question 1 Answer 1-1 Answer 1-2,Answer 1-3,Answer 1-4
Question 2 Answer 2-1 Answer 2-2,Answer 2-3,Answer 2-4
然后,在应用程序代码中拆分,
上的不正确答案列变得很简单,因为它是一个逗号分隔的列表
例如,在PHP中,类似于:
$incorrect = explode(',', $row['Incorrect_answers']);
或者使用Ruby或Python:
incorrect = incorrect_answers.split(',')
您可以使用带有大小写
表达式的聚合函数来透视数据。您可以使用用户定义的变量按问题在每一行上实现行号。您的代码与此类似:
select q.text Question,
max(case when a.is_correct = 1 then a.text end) Correct_answer,
max(case when a.is_correct = 0 and rn=1 then a.text end) Incorrect_Answer1,
max(case when a.is_correct = 0 and rn=2 then a.text end) Incorrect_Answer2,
max(case when a.is_correct = 0 and rn=3 then a.text end) Incorrect_Answer3
from question q
inner join
(
select a.id_question,
a.text,
a.is_correct,
a.id_answer,
@row:=case
when @prevQ=id_question
and is_correct = 0
then @row +1
else 0 end rn,
@prevA:=id_answer,
@prevQ:=id_question
from answer a
cross join (select @row:=0, @prevA:=0, @prevQ:=0)r
order by a.id_question, a.id_answer
) a
on q.id_question = a.id_question
group by q.text
order by a.id_question, a.id_answer
看。这将在单独的列中给出结果:
| QUESTION | CORRECT_ANSWER | INCORRECT_ANSWER1 | INCORRECT_ANSWER2 | INCORRECT_ANSWER3 |
-------------------------------------------------------------------------------------------
| Question 1 | Answer 1-1 | Answer 1-2 | Answer 1-3 | Answer 1-4 |
| Question 2 | Answer 2-1 | Answer 2-2 | Answer 2-3 | Answer 2-4 |
我不熟悉MySQL中的pivot查询,但这是开始谷歌搜索的好地方:)。希望我能帮助更多。。。我相信有人会把答案贴出来的。