通过特定的MAX（字段）组合后返回记录

Question

我在这么多的主题上搜索，但我无法适用于我的查询。 这就是：

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, SUM(CASE WHEN r.user IS NULL THEN 1 ELSE 0 END) to_view 
FROM forum_categories 
JOIN forum_topics ON forum_topics.category_id=forum_categories.id 
LEFT OUTER JOIN (SELECT topic, user FROM forum_visits WHERE user='userA') r ON forum_topics.id=r.topic 
JOIN forum_messages ON forum_messages.topic_id=forum_topics.id 
GROUP BY forum_categories.id 
ORDER BY forum_categories.date 

它的工作原理：唯一的问题是，只返回现场forum_messages.date与MAX的数据; intead，我想用MAX字段返回整行（例如，相应的作者）。

所以我应该返回什么，在更短的话来说，就是：

1. 标题为每个类别; *（此刻起作用）
2. 该类别的主题数量; （此刻起作用）
3. 该类别所有主题的回复数量; （这里有另一个条件，你可以看到，这个答复的计数器是通过带有原始值的消息得到的= 0）（目前这个工作）
4. 最后消息的作者/数据类别（这里有问题：它只能正确返回日期，而不是作者）;
5. 一个标志，指示是否有任何userA尚未检查的主题; （也在此刻工作：如果SUM返回的值高于0，则有一个主题未被查看）
6. 此查询的插入速度尽可能快，因为表格可能非常大;

有关详细信息，这些都是我的实际表：

CREATE TABLE IF NOT EXISTS `forum_categories` (
    `id` int(11) unsigned NOT NULL auto_increment, 
    `title` varchar(255) NOT NULL, 
    `description` varchar(255) NOT NULL, 
    `date` datetime NOT NULL, 
    PRIMARY KEY (`id`) 
) ENGINE=MyISAM AUTO_INCREMENT=12 DEFAULT CHARSET=utf8; 

CREATE TABLE IF NOT EXISTS `forum_topics` (
    `id` int(11) unsigned NOT NULL auto_increment, 
    `category_id` int(11) unsigned NOT NULL, 
    `title` varchar(255) NOT NULL, 
    `author` varchar(255) NOT NULL, 
    `date` datetime NOT NULL, 
    `view` int(11) unsigned NOT NULL default '0', 
    `sticky` tinyint(11) unsigned NOT NULL default '0', 
    PRIMARY KEY (`id`) 
) ENGINE=MyISAM AUTO_INCREMENT=25 DEFAULT CHARSET=utf8;  

CREATE TABLE IF NOT EXISTS `forum_messages` (
    `id` int(11) unsigned NOT NULL auto_increment, 
    `topic_id` int(11) unsigned NOT NULL, 
    `author` varchar(255) NOT NULL, 
    `message` mediumtext NOT NULL, 
    `date` datetime NOT NULL, 
    `original` tinyint(11) unsigned NOT NULL default '0', 
    PRIMARY KEY (`id`) 
) ENGINE=MyISAM AUTO_INCREMENT=29 DEFAULT CHARSET=utf8; 

CREATE TABLE IF NOT EXISTS `forum_visits` (
    `id` int(11) unsigned NOT NULL auto_increment, 
    `topic` int(11) unsigned NOT NULL, 
    `user` varchar(255) NOT NULL, 
    PRIMARY KEY (`id`), 
    UNIQUE KEY `forum_visits_unique_idx` (`topic`,`user`) 
) ENGINE=MyISAM AUTO_INCREMENT=131 DEFAULT CHARSET=utf8; 

希望有人能帮帮我！

Answer 1

我们可以使用查询从您刚才的问题得到消息的作者和消息日期，上面的查询来获取计数器（主题和回复），并且将它们连接在一起：

编辑：这个查询工作（测试）。但它有一点复杂，并有2个子查询，所以我希望别人发布一个更简单的。如果数据库变得非常大，这个可能会变得迟缓。

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, 
t2.author, t2.last_message 

-- first get the counters per category 
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 

-- Then join a query to get last message per category 
JOIN (SELECT forum_categories.id, 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 
      GROUP BY forum_categories.id) t2 
    ON t2.id = forum_categories.id 

GROUP BY forum_categories.id 

Answer 2

为了补充缺少数据的电流输出，我可能会是这样的：

SELECT 
    forum_stats.*, /* just repeat the already pulled columns (expand it if needed) */ 
    forum_messages.* /* and here you may actually want to be more specific as to 
         what else you would like to pull from forum_messages */ 
FROM (
    SELECT 
    forum_categories.id AS category_id, 
    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, 
    MAX(forum_messages.date) AS last_message, 
    SUM(CASE WHEN r.user IS NULL THEN 1 ELSE 0 END) AS to_view, 
    forum_categories.date 
    FROM forum_categories 
    JOIN forum_topics ON forum_topics.category_id=forum_categories.id 
    LEFT OUTER JOIN (
     SELECT topic, user FROM forum_visits WHERE user='userA' 
    ) r ON forum_topics.id=r.topic 
    JOIN forum_messages ON forum_messages.topic_id=forum_topics.id 
    GROUP BY forum_categories.id 
) forum_stats 
    JOIN forum_topics ON forum_topics.category_id=forum_stats.category_id 
    JOIN forum_messages ON forum_messages.topic_id=forum_topics.id 
    AND forum_messages.date=forum_stats.last_message 
ORDER BY forum_stats.date 

当然，这个假设forum_messages.date不只是一个日期，但时间戳和无两条消息可以共享完全相同的时间戳。