Question

我有一个查询，我试图通过最新的postdate或最新的commentdate对每行进行排序。

帖子可能有很多评论，所以我需要获得每行的最大评论日期。如果postdate大于commentdate cdate = postdate else，否则为cdate = max commentdate。

我试图通过以下方式实现这一目标：

CASE WHEN max(wc.date_created) > wp.date_created THEN max(wc.date_created) ELSE wp.date_created END AS cdate

ORDER BY cdate DESC

非常感谢任何帮助。

这是完整的查询（似乎用每行的commentdate填充cdate）：

SELECT DISTINCT wp.p_id, wp.type, wp.value, wp.media, wp.youtube, wp.post_type, wp.tagedpersons, wp.title AS thetitle, wp.url, wp.description, wp.cur_image, wp.uip, wp.likes, wp.userid, wp.posted_by, wp.post AS postdata, wu . * , UNIX_TIMESTAMP( ) - wp.date_created AS TimeSpent, wp.date_created, wp.course, 
CASE WHEN max(wc.date_created) > wp.date_created THEN max(wc.date_created) ELSE wp.date_created END AS cdate
FROM wallposts wp
INNER JOIN wallusers wu ON wu.mem_id = wp.userid
INNER JOIN wallcomments wc ON wc.post_id = wp.p_id
WHERE (
          wp.userid IN (".$matches.") OR 
          (wp.userid IN (".$courses.") AND wp.course = 1)  OR 
          wp.userid =".$user_id." OR
          wp.tagedpersons LIKE '%".$user_id."%' OR
          EXISTS (SELECT * FROM wallcomments 
          WHERE wp.p_id = wallcomments.post_id AND wallcomments.tagedpersons LIKE '%".$user_id."%')
)
GROUP BY wp.p_id
ORDER BY cdate DESC

表结构 - 内部联接墙注释wc.post_id = wp.p_id

CREATE TABLE IF NOT EXISTS `wallposts` (
  `p_id` int(11) NOT NULL AUTO_INCREMENT,
  `post` text NOT NULL,
  `type` varchar(55) NOT NULL,
  `value` int(11) NOT NULL,
  `date_created` int(11) NOT NULL,
  `userid` varchar(255) NOT NULL,
  `posted_by` int(11) NOT NULL,
  `likes` int(11) NOT NULL,
  `media` int(11) NOT NULL,
  `uip` varchar(222) NOT NULL,
  `title` varchar(255) NOT NULL,
  `description` text NOT NULL,
  `url` text NOT NULL,
  `cur_image` text NOT NULL,
  `post_type` tinyint(1) NOT NULL,
  `youtube` text NOT NULL,
  `tagedpersons` varchar(255) NOT NULL,
  `course` int(255) NOT NULL DEFAULT '0',
  `ctimespent` varchar(255) NOT NULL,
  PRIMARY KEY (`p_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=537 ;

CREATE TABLE IF NOT EXISTS `wallcomments` (
  `c_id` int(11) NOT NULL AUTO_INCREMENT,
  `userid` int(11) NOT NULL,
  `comments` text NOT NULL,
  `date_created` int(11) NOT NULL,
  `post_id` int(11) NOT NULL,
  `clikes` int(11) NOT NULL,
  `uip` varchar(222) NOT NULL,
  `tagedpersons` varchar(255) NOT NULL,
  `deleted` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`c_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=349 ;

示例数据：

wp.date_created和wc.date_created（格式） - 1356008534

Answer 1

使它更整洁并将其置于标量函数

CASE WHEN max(wc.date_created) > wp.date_created THEN max(wc.date_created) ELSE wp.date_created END AS cdate

成为

dbo.ReturnDate(wp.p_id) which returns a date for you

然后在您的函数中，您可以执行查询以获取最大日期

返回日期可能包含类似

的内容

CREATE FUNCTION dbo.ReturnDate(@pid int)
RETURNS date

DECLARE @ReturnDate date, @AnotherDate date
SET @ReturnDate  = (SELECT wp.date_created FROM WallPosts wp WHERE wp.p_id = @pid)
SET @AnotherDate = (SELECT MAX(wc.date_created) FROM WallComments wc WHERE p_id = @pid)

IF @AnotherDate > @ReturnDate
SET @ReturnDate = @AnotherDate

RETURN @ReturnDate

注意我没有检查语法

Answer 2

date_created是一个INT，我假设这可以作为SQL服务器中的日期进行转换？

CAST他们都是DATE，例如：

CAST(wp.date_created AS DATE) > CAST(MAX(wc.date_created AS DATE))

所以

CASE WHEN CAST(max(wc.date_created) AS DATE) > CAST(wp.date_created AS DATE) THEN CAST(max(wc.date_created) AS DATE) ELSE CAST(wp.date_created AS DATE) END AS cdate

Answer 3

排序，效果很好.....

评论表上的LEFT JOIN是关键，也改变了

CASE WHEN max(wc.date_created) > wp.date_created THEN max(wc.date_created) ELSE wp.date_created END AS cdate

TO

greatest(max(wp.date_created), COALESCE(max(wc.date_created),0)) AS cdate

以下是最终查询：

SELECT DISTINCT wp.p_id, wp.type, wp.value, wp.media, wp.youtube, wp.post_type, wp.tagedpersons, wp.title AS thetitle, wp.url, wp.description, wp.cur_image, wp.uip, wp.likes, wp.userid, wp.posted_by, wp.post AS postdata, wu . * , UNIX_TIMESTAMP( ) - wp.date_created AS TimeSpent, wp.date_created, COALESCE(max(wc.date_created),0), wp.course, 
greatest(max(wp.date_created), COALESCE(max(wc.date_created),0)) AS cdate
FROM wallposts wp
INNER JOIN wallusers wu ON wu.mem_id = wp.userid
LEFT JOIN wallcomments wc ON wc.post_id = wp.p_id
WHERE (
          wp.userid IN (".$matches.") OR 
          (wp.userid IN (".$courses.") AND wp.course = 1)  OR 
          wp.userid =".$user_id." OR
          wp.tagedpersons LIKE '%".$user_id."%' OR
          EXISTS (SELECT * FROM wallcomments 
          WHERE wp.p_id = wallcomments.post_id AND wallcomments.tagedpersons LIKE '%".$user_id."%' AND deleted = 0)
)
GROUP BY wp.p_id
ORDER BY cdate DESC

MySQL一对多SELECT MAX与CASE获得最佳日期

3 个答案: