mysql - 在 group_concat 中显示空记录

Question

我有两张这样的桌子

profile_answers

+---------+------------+
|      id | class_name |
+---------+------------+
|       1 | Class 1    |
|       2 | Class 2    |
|       3 | Class 1    |
+---------+------------+

教育

+---------+--------------------+------------+
|      id | profile_answers_id |  sample    |
+---------+--------------------+------------+
|       1 | 1                  |     1234   |
|       2 | 1                  |     2334   |
|       3 | 1                  |     3434   |
+---------+------------+--------------------+

我运行了查询，

select educations.profile_answer_id, GROUP_CONCAT(educations.sample) from educations
LEFT JOIN profile_answers ON educations.profile_answers_id = profile_answers.id

我有

+--------+--------------------+-------------+
|      id | sample                          | 
+---------+--------------------+------------+
|       1 | 1234,2334,3434                  |
+---------+------------+--------------------+

其实我想要

+--------+--------------------+-------------+
|      id | sample                          | 
+---------+--------------------+------------+
|       1 | 1234,2334,3434                  |
|       2 | NULL                            |
|       3 | NULL                            |  
+---------+------------+--------------------+

我尝试了 ISNULL、IFNULL 的组合，group_by profile_answers.id它们给了我正确的结果，但结果证明它在运行时间方面非常昂贵

Answer 1

这是我在你之前的问题中的回答...

看起来您缺少 GROUP BY：

select profile_answers.id, GROUP_CONCAT(educations.sample) 
from profile_answers
LEFT JOIN educations ON educations.profile_answers_id = profile_answers.id
GROUP BY profile_answers.id

我还更改了您的 JOIN 以使 profile_answers 表成为您的主表。

祝你好运。