Php mysql是字段的算术运算

我需要一个mysql命令来选择上次推文时间大于或等于（当前时间减去推文数量）的用户

其中：

num_tweet

是一些tweet，比如每小时tweet->3600，每2小时tweet->7200

完整代码：

$now = time();
$sel_users = mysql_query("SELECT * FROM `users` WHERE `status`='1' AND `last_update`<=($now - `num_tweet`)");
$num = @mysql_num_rows($sel_users);
//echo $num;
if($num > 0){
  while($row_users = mysql_fetch_array($sel_users)){
    $sel_tweet = mysql_query("SELECT * FROM `tweets_library` ORDER BY RAND() LIMIT 0,1");
    if(@mysql_num_rows($sel_tweet) > 0){
      $row_tweet    = @mysql_fetch_array($sel_tweet);
      $twitteroauth = new TwitterOAuth($YOUR_CONSUMER_KEY, $YOUR_CONSUMER_SECRET, $row_users['access_token_oauth_token'], $row_users['access_token_oauth_token_secret']);
      $tweet = $twitteroauth->post('statuses/update',array('status' => $row_tweet['msg']));
      if($tweet->id){$now = time();mysql_query("UPDATE `users` SET `last_update`='$now' WHERE `id`='$row_users[id]'");}
    }
  }
}

$now=time（）；
$sel\u users=mysql\u query（“从`users`WHERE`status`='1'和`last\u update`中选择*不在PHP中生成时间并将它们填充到查询中。mysql可以为您这样做：
WHERE last\u update>=（unix\u timestamp（now（））-num\u tweet）
。另外，您的单行样本使用
=
而您的完整样本使用
的原因是什么
$now = time();
$sel_users = mysql_query("SELECT * FROM `users` WHERE `status`='1' AND `last_update`<=($now - `num_tweet`)");
$num = @mysql_num_rows($sel_users);
//echo $num;
if($num > 0){
  while($row_users = mysql_fetch_array($sel_users)){
    $sel_tweet = mysql_query("SELECT * FROM `tweets_library` ORDER BY RAND() LIMIT 0,1");
    if(@mysql_num_rows($sel_tweet) > 0){
      $row_tweet    = @mysql_fetch_array($sel_tweet);
      $twitteroauth = new TwitterOAuth($YOUR_CONSUMER_KEY, $YOUR_CONSUMER_SECRET, $row_users['access_token_oauth_token'], $row_users['access_token_oauth_token_secret']);
      $tweet = $twitteroauth->post('statuses/update',array('status' => $row_tweet['msg']));
      if($tweet->id){$now = time();mysql_query("UPDATE `users` SET `last_update`='$now' WHERE `id`='$row_users[id]'");}
    }
  }
}