Question

我有两个表posts和posts_replies我查询了所有帖子并按时间戳（发布时间）排序。现在我想做以下事情：每当用户对任何帖子做出回复时，我都希望将该帖子放在墙上所以我确实得到了最大的回复＃39;每个帖子的时间戳，并使用特定帖子的回复的最大时间戳订购帖子。

问题是某些帖子没有任何回复所以这篇文章的最大timestamp_of_replies将为NULL所以我想知道：当它不为空时，是否可以通过timestamp_of_replies对结果进行排序;当它是非空时，可以通过post_timestamp对结果进行排序NULL。

我的查询：

SELECT 
    posts.*,
    u1.id as user_id, 
    u1.avatar, 
    u1.username as poster_username, 
    u2.username as posted_username , 
    posts.timestamp, 
    f1.recent_reply_time
FROM 
    posts 
INNER JOIN 
    users u1 
    ON posts.poster_id = u1.id  
INNER JOIN 
    users u2 
    ON posts.posted_id = u2.id
LEFT JOIN (
    SELECT 
        max(timestamp) as recent_reply_time, 
        post_id 
    FROM 
        posts_replies 
    GROUP BY post_id) f1 
    ON f1.post_id = posts.id
order by 
    f1.recent_reply_time DESC

注意：order by f1.recent_reply_time, posts.timestamp DESC没有给我正确的结果

Answer 1

MySQL有一个IF()函数，您也可以在ORDER BY子句中使用它：

ORDER BY IF(column IS NULL, othercolumn, column)

在你的情况下，它将是：

ORDER BY IF(f1.recent_reply_time IS NULL, posts.timestamp, f1.recent_reply_time)

Answer 2

SORTING易于阅读和维护，如果您根据自己选择的内容制作它。最好在您的选择中添加一些sortkey并在ORDER BY中使用它。

我使用COALESCE来处理空值。通过使用适当的COALESCE参数，使至少具有一个NOT NULL值的排序键

 SELECT posts.*,
       u1.id as user_id,
       u1.avatar,
       u1.username as poster_username, 
       u2.username as posted_username,
       posts.timestamp,
       f1.recent_reply_time,
       COALESCE(f1.recent_reply_time,posts.timestamp) as sortkey
    FROM posts 
    INNER JOIN users u1 ON posts.poster_id = u1.id  
    INNER JOIN users u2 ON posts.posted_id = u2.id
    LEFT JOIN (SELECT max(timestamp) as recent_reply_time, post_id FROM posts_replies GROUP BY post_id) f1 ON f1.post_id = posts.id
    order by sortkey DESC

Answer 3

使用COALESCE：

SELECT posts.*, u1.id as user_id, u1.avatar, u1.username as poster_username, 
       u2.username as posted_username , posts.timestamp, f1.recent_reply_time
FROM posts 
    INNER JOIN users u1 ON posts.poster_id = u1.id  
    INNER JOIN users u2 ON posts.posted_id = u2.id
    LEFT JOIN (SELECT max(timestamp) as recent_reply_time, post_id 
               FROM posts_replies 
               GROUP BY post_id) f1 ON f1.post_id = posts.id
ORDER BY COALESCE(f1.recent_reply_time,[alternative value (CURDATE())]) DESC

如果某些值为null，如何按列排序查询？

3 个答案: