$result = mysql_query("SELECT id FROM db_topics WHERE mainnumber='$mainid'");
$ids = Array();
while($row = mysql_fetch_array($result)){
  $ids[] = $row['id'];
}
$sql = "DELETE FROM t1.*, t2.* FROM db_thread t1, db_thread t2 WHERE "
     . "t1.threadid IN (" . implode(', ', $ids) . ") AND "
     . "t1.threadid=t2.id";
mysql_query($sql);

/*
	while ($row = mysql_fetch_array($result)) {	
		$topicid = $row['id'];
		mysql_query("DELETE from db_thread WHERE threadid='$topicid'") or die(mysql_error());
		mysql_query("DELETE from db_topics WHERE id='$topicid'") or die(mysql_error());
	}
*/