Question

I have a query where I’m trying to sort per row either by the latest postdate or the latest commentdate.

A post could have many comments so I need to get the max comment date for each row. If the postdate is greater than the commentdate cdate = postdate else if not cdate = max commentdate.

I am trying to achieve this using:

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

Any help would be greatly appreciated.

Here is the full query (it seems to populate cdate with the commentdate for every row):

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

Table structure – INNER JOIN wallcomments wc ON 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 ;

Sample data:

wp.date_created and wc.date_created (format) – 1356008534

Answer 1

Sorted, works great…..

LEFT JOIN on the comments table was the key and also changed

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

Here is the final query:

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