如何从多态MySQL表中检索数据

Question

我在MySQL中有三个表：application,questions和questions_answer。

application店user_id; questions商店question_id和type of questions（即名称，身份证，学校名称）; questions_answer存储参考user_id和question_id的answers。

据我所知，这种类型的关联称为多态关联。现在我迷失了如何从不同的question_id中检索数据，并将它们改为列标题。

我希望这是有道理的。

编辑：

为了说明，这里是各自的表：

application：

user_id   name 
------------------------------- 
100    Leon Barnacles 
101    Richard Kennard 
102    Fareeza Salleh 

questions：

question_id  question_name 
--------------------------------------------- 
20    NRIC 
21    Have you ever applied to TFM? 
22    What's your current GPA? 
23    Name of school 

questions_answer：

question_id  user_id  answer 
------------------------------------------------ 
20    100   880808-06-8990 
20    100   900990-14-0911 
23    102   SMK Taman Pandamaran 

我希望什么检索：

Name    NRIC    Name of school 
------------------------------------------------------------ 
Leon Barnacles  880808-06-8990  
Richard Kennard  900990-14-0911 
Fareeza Salleh       SMK Taman Pandamaran 

Answer 1

你需要的是一个PIVOT型功能，MySQL不支持PIVOT如SQL Server或Oracle。您可以使用下面的脚本使用max and case问题上表中的数据

select group_concat(concat('max(case when question_id = ''', question_id, ''' then 
      answer end) as `', question_name ,'`')) into @sql from tfm_questions; 

set @sql = 
concat('select full_name, ', @sql, ' from 
      (
      select a.full_name, q.question_name, an.answer, q.question_id 
      from tfm_application a 
      inner join tfm_questions_answer an on a.user_id = an.user_id 
      inner join tfm_questions q on an.question_id = q.question_id 
     ) x 
     group by full_name'); 


select @sql; 

PREPARE stmt FROM @sql; 
EXECUTE stmt; 
DEALLOCATE PREPARE stmt; 

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