Group by 也是 order by？

Question

两个表：话题评论（有一个主题字段，所以我知道评论是在哪个主题）

我只想获取每个主题中的最新评论（日期字段中的最大值），然后以这种方式对主题进行排序。

我试过的查询：

SELECT User, Topic, Date 
FROM   Comments 
GROUP BY Topic 
ORDER BY Date DESC

Answer 1

试试这个，改进的答案：

SELECT `User`,
       temp.`Topic`,
       temp.`Date`
FROM (
       SELECT `Topic`,
              MAX(`Date`) `Date`
       FROM `Comments`
       GROUP BY `Topic`
       ORDER BY MAX(`Date`) DESC
      ) temp
INNER JOIN `Comments`
     USING (`Topic`, `Date`)

Answer 2

Return 如果同一主题的其他行没有更晚的日期，则为一行。

SELECT User, Topic, Date 
FROM   Comments c1
where not exists (select 1 from Comments c2
                  where c2.topic = c1.topic
                    and c2.date > c1.date)
order by date desc

Group by 也是 order by？

Group by and also order by?

mysql

sql

sql-order-by