如何编写将特定行值转换为新列的MySQL查询？

Question

这是期望的结果：

sr no. | fullname      | email  
----------------------------------------
1.     | demo user     | demo@test.com 
2.     | sagar panchal | sagar@gmail.com

我试过了：

SELECT *
FROM `ds_tbl_event_question` q
LEFT JOIN `ds_tbl_event_participant` p ON q.event_id = p.event_id
AND p.question_id = q.event_question_id
LEFT JOIN `ds_events` e ON e.event_id = q.event_id
WHERE (
q.field_type = 'email'
OR q.field_type = 'full_name'
)
AND e.store_code = 'H9OCEN'
LIMIT 0 , 30

这给出了输出，如下图所示：

Answer 1

似乎只有字段created_date将您要从中获取full_name和email的记录联系在一起。鉴于此，您可以按此created_date进行分组，并使用聚合函数（例如min）仅考虑answer - 具有特定field_type - 值的记录的值：

select
  min(CASE field_type WHEN 'full_name' THEN answer ELSE NULL END) as 'fullname',
  min(CASE field_type WHEN 'email' THEN answer ELSE NULL END) as 'email'   
FROM `ds_tbl_event_question` q
     LEFT JOIN `ds_tbl_event_participant` p ON q.event_id = p.event_id AND p.question_id = q.event_question_id
     LEFT JOIN `ds_events` e ON e.event_id = q.event_id
WHERE (
      q.field_type = 'email'
   OR q.field_type = 'full_name'
   )
   AND e.store_code = 'H9OCEN'
group by created_date

请注意，聚合函数（此处为：min）是必需的，因为select-clause不能包含除group by子句中列出的属性之外的其他属性，除非在聚合函数中使用。

在上面的查询中，我尝试尽可能少地调整您的查询。实际上可以进一步缩短查询：当您在左侧值使用WHERE条件时，左连接实际上变成了连接;并且field_type上的条件是多余的，因为case - 语句根本不考虑任何其他值：

select
  min(CASE field_type WHEN 'full_name' THEN answer ELSE NULL END) as 'fullname',
  min(CASE field_type WHEN 'email' THEN answer ELSE NULL END) as 'email'   
FROM `ds_tbl_event_question` q
     JOIN `ds_tbl_event_participant` p ON q.event_id = p.event_id AND p.question_id = q.event_question_id
     JOIN `ds_events` e ON e.event_id = q.event_id AND e.store_code = 'H9OCEN'
group by created_date

Answer 2

尝试这样的事情：

SELECT full_names.answer AS full_name,
       emails.answer AS email FROM
(SELECT created_date
 FROM ds_tbl_event_question
 GROUP BY created_date) created_dates
JOIN 
(SELECT created_date, answer
 FROM ds_tbl_event_question
 WHERE field_type = 'full_name') full_names
ON full_names.created_date = created_dates.created_date
JOIN 
(SELECT created_date, answer
 FROM ds_tbl_event_question
 WHERE field_type = 'email') emails
ON emails.created_date = created_dates.created_date

请参阅Rextester演示：http://rextester.com/OBQ69221

Answer 3

似乎关联的唯一方法是创建日期：

选择q.answer作为全名，       （从ds_tbl_event_question中选择答案，其中field_type =＆＃39; email＆＃39;和created_date = q.created_date）作为电子邮件 来自ds_tbl_event_question q其中q.field_type =＆＃39; full_name＆＃39;;