一切正常，但如果我添加groupbymessagesuserone到orderbymessagedatedesc，我会得到一个错误 Trying to get property of non-object in ... . 我哪里错了？…，你能纠正代码，看看它看起来怎么样。。。拜托，我需要这个，我一整天都在忙。。请不要太多描述，我的英语不是最好的。谢谢。
php版本：7
table：
用户
用户名、电子邮件、密码、名字、姓氏、头像
住宅用户
用户\住宅ID，住宅ID，州，市
用户\u消息
messagesid，messagesUserId，messagesUserWoId，message，messagedate，messageread

if(isset($_SESSION['userID'])){
$user = $_SESSION['userID'];

$sql = "SELECT messagesusertwoID, messagesuseroneID, messagedate,
messageread, SUM(messageread) AS messageread_sum, 
userID, firstname, lastname, avatar, state, city  FROM users_messages 
LEFT JOIN users ON userID = messagesuseroneID 
LEFT JOIN users_residence ON userresidenceID = messagesuseroneID 
WHERE messagesusertwoID = '$user' 
GROUP BY messagesuseroneID ORDER BY messagedate DESC ";
                 $result = $con->query($sql);

                 if ($result->num_rows > 0) {
                 // output data of each row
                 while($row = $result->fetch_assoc())
                     {

                     echo   ''.$row["avatar"].'
                             '.$row["userID"].'
                             '.$row["firstname"].'
                             '.$row["lastname"].'
                             '.$row["state"].'
                             '.$row["city"].'
                             '.$row["messageread_sum"].'';

                 }
                 } else {
                 echo "0 results";
                 }
                 }