如何获取用户之间对话的最新消息?(聊天系统）

我正在构建一个私有消息系统。

我想根据通话时间得到最新消息。现在它按时间顺序显示消息，但我希望它按逆时间顺序显示，最新消息在顶部，等等

表

SELECT `messages`.`id`,
    `messages`.`sender`,
    `messages`.`receiver`,
    `messages`.`message`,
    `messages`.`datetime`,
    `messages`.`classifield_id`
FROM `augl`.`messages`;

功能

public function getMessageList()
{
    $current_user = $this->accounts->getUserId();
    $query = $this->db->query("SELECT * FROM(
                                SELECT id, LEAST(receiver) as receiver, LEAST(sender) AS sender, message, datetime, classifield_id 
        FROM messages
        WHERE sender='$current_user' OR receiver = '$current_user' 
        Group By (if(sender > receiver,  sender, receiver))
               , (if(sender > receiver,  receiver, sender))
        ) 
            as id");
    $result =  $query->result();
    //var_dump($result); die;
    foreach ($result as $messages )
    {
        $return[$messages->id] = $messages;
        if ($messages->receiver === $current_user)
        {
            $return[$messages->id]->userr = $this->accounts->getUsername($messages->sender);
            $return[$messages->id]->userid = $messages->sender;
            //$return[$messages->id]->message = $messages->message;
        }
        else
        {
            $return[$messages->id]->userr = $this->accounts->getUsername($messages->receiver);
            $return[$messages->id]->userid = $messages->receiver;
            //$return[$messages->id]->message = $messages->message;
        }

    }
    return $return;
}