我处于死胡同。 SQL查询以获取最后回复的用户名和时间戳

Question

这是我试图开始工作的查询。

SELECT 
    t.id AS topic_id, 
    t.title AS topic_title, 
    t.content AS topic_content, 
    t.created_at AS topic_created_at, 
    t.updated_at AS topic_updated_at, 
    t.user_id AS topic_user_id, 
    c.id AS comment_id, 
    c.content AS comment_content, 
    c.created_at AS comment_created_at, 
    max_c.username AS comment_username, 
    u.username AS topic_username 
FROM 
    topics t 
JOIN 
    (SELECT 
     c2.topic_id, c2.created_at, u2.username 
    FROM 
     comments c2 
    JOIN 
     users u2 ON c2.user_id = u2.id 
    JOIN 
     topics t2 ON c2.topic_id = t2.id 
    ORDER BY 
     c2.created_at desc) max_c ON t.id = max_c.topic_id 
JOIN 
    comments c ON max_c.created_at = c.created_at 
JOIN 
    users u ON u.id = t.user_id 
ORDER BY 
    c.created_at DESC 

很肯定的查询，这部分是不正确的：

SELECT 
    c2.topic_id, c2.created_at, u2.username 
FROM 
    comments c2 
JOIN 
    users u2 ON c2.user_id = u2.id 
JOIN 
    topics t2 ON c2.topic_id = t2.id 
ORDER BY 
    c2.created_at desc 

该查询当前显示如下。但我想通过created_at或任何合适的组合，所以我们只能得到最新的回复话题。

如果你能帮助这将是惊人的，我花了大约5个小时努力，到目前为止写这个...

我下面附上我的表迁移。

# Dump of table comments 
# ------------------------------------------------------------ 

CREATE TABLE `comments` (
    `id` int(11) NOT NULL AUTO_INCREMENT, 
    `content` varchar(255) DEFAULT NULL, 
    `user_id` int(11) NOT NULL, 
    `topic_id` int(11) DEFAULT NULL, 
    `updated_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP, 
    `created_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP, 
    PRIMARY KEY (`id`), 
    KEY `user_id` (`user_id`), 
    KEY `comments_ibfk_2` (`topic_id`), 
    CONSTRAINT `comments_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`), 
    CONSTRAINT `comments_ibfk_2` FOREIGN KEY (`topic_id`) REFERENCES `topics` (`id`) ON DELETE CASCADE ON UPDATE CASCADE 
) ENGINE=InnoDB DEFAULT CHARSET=utf8; 



# Dump of table topics 
# ------------------------------------------------------------ 

CREATE TABLE `topics` (
    `id` int(11) NOT NULL AUTO_INCREMENT, 
    `title` varchar(255) DEFAULT NULL, 
    `content` text, 
    `user_id` int(11) NOT NULL, 
    `created_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP, 
    `updated_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP, 
    PRIMARY KEY (`id`), 
    KEY `user_id` (`user_id`), 
    CONSTRAINT `topics_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8; 



# Dump of table users 
# ------------------------------------------------------------ 

CREATE TABLE `users` (
    `id` int(11) NOT NULL AUTO_INCREMENT, 
    `username` varchar(255) DEFAULT 'NOT NULL', 
    `email` varchar(255) DEFAULT 'NOT NULL', 
    `password` char(60) DEFAULT 'NOT NULL', 
    `admin` int(11) NOT NULL DEFAULT '0', 
    `created_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP, 
    PRIMARY KEY (`id`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8; 

Answer 1

如果我理解你的问题，你所遇到的问题是子查询应返回只有最后一个用户名和时间戳，但返回所有来代替。在这种情况下，您可以在订单后使用LIMIT 1以获得所需的结果。

SELECT c2.topic_id, c2.created_at,u2.username 
    FROM comments c2 
    JOIN users u2 ON c2.user_id = u2.id 
    JOIN topics t2 ON c2.topic_id = t2.id 
    ORDER BY c2.created_at desc 
    LIMIT 1 

Answer 2

请尝试以下...

SELECT topics.id AS topic_id, 
     topics.title AS topic_title, 
     topics.content AS topic_content, 
     topics.created_at AS topic_created_at, 
     topics.updated_at AS topic_updated_at, 
     topicsUsers.user_id AS topic_user_id, 
     topicsUsers.username AS topic_username, 
     comments.id AS comment_id, 
     comments.content AS comment_content, 
     maxCreatedAt AS comment_created_at, 
     commentsUsers.user_id AS comment_user_id 
     commentsUsers.username AS comment_username, 
FROM topics 
JOIN (SELECT topic_id AS topic_id, 
       MAX(comments.created_at) AS maxCreatedAt, 
     FROM comments 
     GROUP BY topic_id 
    ) maxCreatedAtFinder ON topics.id = maxCreatedAtFinder.topic_id 
JOIN comments ON maxCreatedAtFinder.maxCreatedAt = comments.created_at 
      AND maxCreatedAtFinder.topic_id = topics.id 
JOIN users AS topicsUsers ON users.id = topics.user_id 
JOIN users AS commentsUsers ON users.id = comments.user_id 
ORDER BY maxCreatedAt DESC 

，尽我所能从你的问题告诉，你正在寻找显示每个topic的详细信息（包括对的user详细信息列表user谁发起Topic）以及Topic的最新comment以及user谁发布了comment的详细信息。

我的声明遵循与您提供的声明类似的结构。我已从子查询中删除JOIN至topics，因为topic_id是topics的唯一细节，我们希望子查询可以从comments获得topic_id。

获得的user的谁发布的最新comment为Topic将分组复杂化，并导致更多的细节被联接比如果我们加入Comments到Users子查询外执行。所有子查询确实需要获取每个Topic的最近值created_at。因此，我已将JOIN删除为Users以及相关的字段选择。

在主要发言中，我一直保持着子查询的INNER JOIN到topics，从而有效地追加的created_at最大值从comments从topics其相应的记录。

我再取所得的数据集和以这样的方式将其加入comments一个topic和从comments的created_at其相关联的最近的值具有每个comment的该topic与内容即created_at值附加到它。请注意，虽然不太可能同时创建属于topic的两个comments，并因此具有相同的值created_at。这两个记录将被加入到正在形成的数据集中。在没有相反指示的情况下，我认为这是理想的行为，并且已经允许。

我再取所得的数据集和JOIN它的users两个实例，一个基于所述topic_id以及基于所述user_id为comment另一个。这具有附加到我们的数据集中的每个记录的效果，useruser谁创建Topic和user谁发布了最新的comment（s）的细节。

然后，这个最终数据集按照时间顺序排序，从最近的记录开始。

然后选择所需的字段并由语句返回。

如果您有任何问题或意见，请随时发布相应评论。