我處於死衚衕。 SQL查詢以獲取最後回覆的用戶名和時間戳

Question

這是我試圖開始工作的查詢。

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 

很肯定的查詢，這部分是不正確的：

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 

該查詢當前顯示如下。但我想通過created_at或任何合適的組合，所以我們只能得到最新的回覆話題。

如果你能幫助這將是驚人的，我花了大約5個小時努力，到目前爲止寫這個...

我下面附上我的表遷移。

# 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

如果我理解你的問題，你所遇到的問題是子查詢應返回只有最後一個用戶名和時間戳，但返回所有來代替。在這種情況下，您可以在訂單後使用LIMIT 1以獲得所需的結果。

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

請嘗試以下...

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 

，盡我所能從你的問題告訴，你正在尋找顯示每個topic的詳細信息（包括對的user詳細信息列表user誰發起Topic）以及Topic的最新comment以及user誰發佈了comment的詳細信息。

我的聲明遵循與您提供的聲明類似的結構。我已從子查詢中刪除JOIN至topics，因爲topic_id是topics的唯一細節，我們希望子查詢可以從comments獲得topic_id。

獲得的user的誰發佈的最新comment爲Topic將分組複雜化，並導致更多的細節被聯接比如果我們加入Comments到Users子查詢外執行。所有子查詢確實需要獲取每個Topic的最近值created_at。因此，我已將JOIN刪除爲Users以及相關的字段選擇。

在主要發言中，我一直保持着子查詢的INNER JOIN到topics，從而有效地追加的created_at最大值從comments從topics其相應的記錄。

我再取所得的數據集和以這樣的方式將其加入comments一個topic和從comments的created_at其相關聯的最近的值具有每個comment的該topic與內容即created_at值附加到它。請注意，雖然不太可能同時創建屬於topic的兩個comments，並因此具有相同的值created_at。這兩個記錄將被加入到正在形成的數據集中。在沒有相反指示的情況下，我認爲這是理想的行爲，並且已經允許。

我再取所得的數據集和JOIN它的users兩個實例，一個基於所述topic_id以及基於所述user_id爲comment另一個。這具有附加到我們的數據集中的每個記錄的效果，useruser誰創建Topic和user誰發佈了最新的comment（s）的細節。

然後，這個最終數據集按照時間順序排序，從最近的記錄開始。

然後選擇所需的字段並由語句返回。

如果您有任何問題或意見，請隨時發佈相應評論。