php - 根据日期获取最新的帖子

Question

当我有两个表时如何显示最新的帖子，这两个表都包含名为 creation_date 的列

如果我所要做的只是根据 posts created_on 值获取最新的帖子，这将很简单，但是如果帖子包含回复，我需要将其纳入等式。如果帖子有更新的回复，我想获取回复 created_on 值，但也获取帖子 post_id 和主题。

posts 表结构:

CREATE TABLE `posts` (
  `post_id` bigint(20) unsigned NOT NULL auto_increment,
  `cat_id` bigint(20) NOT NULL,
  `user_id` bigint(20) NOT NULL,
  `subject` tinytext NOT NULL,
  `comments` text NOT NULL,
  `created_on` datetime NOT NULL,
  `status` varchar(10) NOT NULL default 'INACTIVE',
  `private_post` varchar(10) NOT NULL default 'PUBLIC',
  `db_location` varchar(10) NOT NULL,
  PRIMARY KEY  (`post_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=7 ;

回复表结构:

CREATE TABLE `replies` (
  `reply_id` bigint(20) unsigned NOT NULL auto_increment,
  `post_id` bigint(20) NOT NULL,
  `user_id` bigint(20) NOT NULL,
  `comments` text NOT NULL,
  `created_on` datetime NOT NULL,
  `notify` varchar(5) NOT NULL default 'YES',
  `status` varchar(10) NOT NULL default 'INACTIVE',
  `db_location` varchar(10) NOT NULL,
  PRIMARY KEY  (`reply_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=5 ;

到目前为止，这是我的查询。我已经删除了提取日期的尝试。

$strQuery = "SELECT posts.post_id, posts.created_on, replies.created_on, posts.subject ";
$strQuery = $strQuery."FROM posts ,replies ";
$strQuery = $strQuery."WHERE posts.post_id = replies.post_id ";
$strQuery = $strQuery."AND posts.cat_id = '".$row->cat_id."'";

Answer 1

$strQuery = "
  SELECT posts.post_id, GREATEST(posts.created_on, replies.created_on)  AS latestDate, posts.subject
  FROM posts, replies
  WHERE posts.post_id = replies.post_id
  AND posts.cat_id = {$row->cat_id}
  GROUP BY posts.post_id
  ORDER BY latestDate DESC;
";

更新:再看一遍，上面的内容实际上是不正确的，因为它不包括那些还没有任何回复的帖子。更正确的做法是:

$strQuery = "
  SELECT posts.post_id, GREATEST(posts.created_on, replies.created_on) AS latestDate,
  FROM posts
  LEFT JOIN replies ON (posts.post_id = replies.post_id)
  WHERE posts.cat_id = {$row->cat_id}
  GROUP BY posts.post_id
  ORDER BY latestDate DESC
  LIMIT 0,1;
";