分组时出现问题

Question

我有这个 MySql 查询：

SELECT forum_categories.title, forum_messages.author, forum_messages.date AS last_message
FROM forum_categories
JOIN forum_topics ON forum_topics.category_id=forum_categories.id 
JOIN forum_messages ON forum_messages.topic_id=forum_topics.id
WHERE forum_categories.id=6
ORDER BY forum_categories.date ASC

输出如下：

Welcome     daniel      2010-07-09 22:14:49
Welcome     daniel      2010-06-29 22:14:49
Welcome     luke        2010-08-10 20:12:20
Welcome     skywalker   2010-08-19 22:12:20
Welcome     delicious   2010-10-09 19:12:20
Welcome     daniel      2011-11-05 23:12:20
Welcome     pierre      2011-11-05 23:12:22

现在，我想使用最大日期对其进行分组。所以查询变成：

SELECT forum_categories.title, forum_messages.author, forum_messages.date AS last_message, MAX(forum_messages.date)
FROM forum_categories
JOIN forum_topics ON forum_topics.category_id=forum_categories.id 
JOIN forum_messages ON forum_messages.topic_id=forum_topics.id
WHERE forum_categories.id=6
GROUP BY forum_categories.id
ORDER BY forum_categories.date ASC

完美！获取的日期正确：2011-11-05 23:12:22（该分组的最大日期）。但我也从这种行为中获取该行的对应用户（在示例中pierre）。

但还需要另一件事。

为什么？它没有选择整行？我该如何解决这个问题？干杯

编辑

事实上，我应该将此分组应用于整个查询：

SELECT forum_categories.title, COUNT(DISTINCT forum_topics.id) AS total_topics, SUM(CASE WHEN forum_messages.original=0 THEN 1 ELSE 0 END) AS total_replies, forum_messages.author, MAX(forum_messages.date) AS last_message
FROM forum_categories
JOIN forum_topics ON forum_topics.category_id=forum_categories.id 
JOIN forum_messages ON forum_messages.topic_id=forum_topics.id
GROUP BY forum_categories.id 
ORDER BY forum_categories.date

Answer 1

试试这个（让我知道它是否有效）：

SELECT forum_categories.title, forum_messages.author, 
   forum_messages.date AS last_message
FROM forum_categories
JOIN forum_topics ON forum_topics.category_id=forum_categories.id
JOIN forum_messages ON forum_messages.topic_id=forum_topics.id
JOIN (SELECT MAX(m.date) as date, top.category_id  
     FROM forum_messages m
     JOIN forum_topics top ON m.topic_id = top.id
     GROUP BY top.category_id) as t
  ON t.category_id = forum_topics.category_id AND t.date = forum_messages.date
WHERE forum_categories.id=6
GROUP BY forum_categories.id

这是第二个选项：

SELECT forum_categories.title, forum_messages.author, 
       forum_messages.date AS last_message
FROM forum_categories
JOIN forum_topics ON forum_topics.category_id=forum_categories.id
JOIN forum_messages ON forum_messages.topic_id=forum_topics.id
WHERE forum_categories.id=6
AND forum_messages.date = (SELECT MAX(date)
                           FROM forum_messages t
                           WHERE t.topic_id = forum_topics.id)
GROUP BY forum_categories.id
ORDER BY forum_categories.date ASC

Answer 2

您需要在 GROUP BY 子句中包含 forum_categories.title 和 forum_messages.author，因为它们不是聚合函数。将 GROUP BY 视为表示结果集中的哪些值应该“挤”到一行中。

此外，您很可能希望按结果集中的列进行排序，例如last_message（而不是forum_messages.date）

所以：

SELECT forum_categories.title, forum_messages.author, forum_messages.date AS last_message,            MAX(forum_messages.date)
FROM forum_categories
JOIN forum_topics ON forum_topics.category_id=forum_categories.id 
JOIN forum_messages ON forum_messages.topic_id=forum_topics.id
WHERE forum_categories.id=6
GROUP BY forum_categories.title, forum_messages.author
ORDER BY last_message ASC