[英]SELECT query getting rows of table as columns in MySQL
I have a MySQL database with two tables for questions and answers. 我有一个MySQL数据库,有两个表格用于问答。 Each question has a correct answer and three incorrect answers.
每个问题都有正确的答案和三个不正确的答案。 There are always four answers for every question and only one is correct.
每个问题总有四个答案,只有一个是正确的。
The tables are: 表格是:
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;
I need help with a select query. 我需要一个选择查询的帮助。 I would like to get a table with questions in the rows and the four answers as columns (first the correct one and then the other three).
我希望得到一个表格,其中包含行中的问题和四个答案作为列(首先是正确的,然后是其他三个)。 So far I was able to get an output like this:
到目前为止,我能够得到这样的输出:
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
...
How I can get the following result? 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
You can pivot the data by using an aggregate function with a CASE
expression. 您可以使用带有
CASE
表达式的聚合函数来旋转数据。 You can use user-defined variables to implement a row number on each row by question. 您可以使用用户定义的变量逐行实现每行的行号。 Your code will be similar to this:
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
See SQL Fiddle with Demo . 请参阅SQL Fiddle with Demo 。 This gives the result in separate columns:
这会在单独的列中显示结果:
| 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 |
Constructing a dynamic pivot query for this is a lot of work. 为此构建动态数据透视查询是很多工作。 Instead, what I would probably do is use MySQL's
GROUP_CONCAT()
aggregate function to create a comma-separated list of the Incorrect_answer
fields, while separating out the Correct_Answer
as its own column: 相反,我可能会做的是使用MySQL的
GROUP_CONCAT()
聚合函数来创建一个以逗号分隔的Incorrect_answer
字段列表,同时将Correct_Answer
分离为它自己的列:
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`
The result this produces as received by your application code looks like: 应用程序代码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
It then becomes trivial in your application code to split the Incorrect_answers
column on the ,
since it is a comma-separated list. 然后,在您的应用程序代码中分割
Incorrect_answers
列就变得微不足道,
因为它是一个以逗号分隔的列表。
In PHP for example, something like : 例如,在PHP中,类似于:
$incorrect = explode(',', $row['Incorrect_answers']);
Or in Ruby or Python: 或者在Ruby或Python中:
incorrect = incorrect_answers.split(',')
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