Mysql 多个分组依据以计数最小元素？

假设在MySQL中，我有一个表

comments

，其中有字段

post\u id、author\u id、date

和

comment

如果我想要每个帖子的第一条评论，我可以这样做

SELECT post_id, author_id, comment, MIN(date) AS comment_date 
FROM comments 
GROUP BY post_id;

SELECT author_id, COUNT(*) AS total_comments 
FROM comments 
GROUP BY author_id 
ORDER BY total_comments DESC;

如果我想得到评论最多的作者，我可以这样做

SELECT post_id, author_id, comment, MIN(date) AS comment_date 
FROM comments 
GROUP BY post_id;

SELECT author_id, COUNT(*) AS total_comments 
FROM comments 
GROUP BY author_id 
ORDER BY total_comments DESC;

我想把这些结合起来，回答“哪些作者发表了最早的评论？”这个问题，我该怎么做呢？

这是一个坏SQL：

SELECT post_id, author_id, comment, MIN(date) AS comment_date 
FROM comments 
GROUP BY post_id;

SELECT

列和

groupby

不一致。仅仅因为您的数据库恰好允许语法，并不意味着它可以满足您的需要

要回答这个问题：

“哪些作者发表了最早的评论？”

第一个查询的正确形式是：

select c.*
from comments c
where c.date = (select min(c2.date)
                from comments c2
                where c2.post_id = c.post_id
               );

还有其他方法可以编写此逻辑，特别是使用窗口函数

那么您只需要聚合：

select author_id, count(*)
from comments c
where c.date = (select min(c2.date)
                from comments c2
                where c2.post_id = c.post_id
               )
group by author_id
order by count(*) desc;

注：这假设最小日期确实对应于“第一条评论”，即使多个作者在同一日期发表评论