Mysql 将行合并为一行，并根据id对行进行计数

我有三张桌子

post_表

id_post | post_text                      
1       | great view                    
2       | happy breakfast           
3       | good night everybody 

id_comment | comment_text         | id_post      
1          | that's amazing       | 1         
2          | of course, the best  | 1       
3          | wish me there        | 1     
4          | yes, happy breakfast | 2    
5          | hehe                 | 2

注释表

id_post | post_text                      
1       | great view                    
2       | happy breakfast           
3       | good night everybody 

id_comment | comment_text         | id_post      
1          | that's amazing       | 1         
2          | of course, the best  | 1       
3          | wish me there        | 1     
4          | yes, happy breakfast | 2    
5          | hehe                 | 2

附上图片

id_picture | picture_name | id_post  
1          | pict_1       | 1  
2          | pict_2       | 1  

我想进行一个查询，使视图如下所示：

id_post | post_text           | picture_name   | comment_count  
1       | great view          | pict_1, pict_2 | 3  
2       | happy breakfast     | null           | 2  
3       | goodnight everybody | null           | 0  

select a.id_post, a.post_text, b.picture_name, count(c.id_comment) as comment_count
from post_table left join
     attach_picture
     on a.id_post=b.id_post left join
     comment_table c
     on a.id_post=c.id_post
group by a.id_post

我这样写查询：

id_post | post_text           | picture_name   | comment_count  
1       | great view          | pict_1, pict_2 | 3  
2       | happy breakfast     | null           | 2  
3       | goodnight everybody | null           | 0  

select a.id_post, a.post_text, b.picture_name, count(c.id_comment) as comment_count
from post_table left join
     attach_picture
     on a.id_post=b.id_post left join
     comment_table c
     on a.id_post=c.id_post
group by a.id_post

查询结果为：

id_post | post_text           | picture_name  | comment_count  
1       | great view          | pict_1        | 6  
2       | happy breakfast     | null          | 2  
3       | goodnight everybody | null          | 0  

结果是，

picture\u name

只捕获1个

picture\u name

即使是

id\u post

也有超过1个

picture\u name

，并且

注释计数

显示

picture\u name

的数量

---

有人能帮我解决问题吗？

您可以随时修改您的查询以执行您想要的操作：

select pt.id_post, pt.post_text,
       group_concat(distinct ap.picture_name) as picture_names,
       count(distinct c.id_comment) as comment_count
from post_table pt left join
     attach_picture ap
     on pt.id_post = ap.id_post left join
     comment_table c
     on pt.id_post = c.id_post
group by pt.id_post;

这个查询所做的工作比它需要的要多，因为您正在沿着两个不同的维度连接帖子。因此，对于每一篇文章，您将得到所有评论和图像的笛卡尔积。如果你对一个给定的用户只有几条评论和帖子，那么这种方法就可以了。如果每个都有数千个，那么这可能会变得相当低效。在这种情况下，解决方案是在进行连接之前进行聚合

select a.id_post, a.post_text, GROUP_CONCAT(b.picture_name),  (select count(id) from comment_table where id_post = a.id)  as comment_count
from post_table a
left join attach_picture on a.id_post=b.id_post 
left join comment_table c on a.id_post=c.id_post
group by a.id_post