问题分组

Question

当我有这样的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 

和输出是如下：

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MAX日化集团的。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！所需日期是正确的：2011-11-05 23:12:22（该分组的最大日期）。但我也从这种行为方面得到相应的线用户（在示例pierre）。

但它需要另一个。

为什么？它不选择整行？我该如何解决这个问题？干杯

编辑

其实，我应该申请这个分组到这整个查询：

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（让我知道如果它的工作原理）：

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

您需要包括forum_categories.title和forum_messages.author在GROUP BY子句，因为它们不是聚合函数。可以想象GROUP BY表示结果集中的哪些值应该一起“排列”为一行。

而且你很可能会想列在结果集中订购，像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