关于PHP-数据库的 数据读取，Trying to get property 'num_rows' of non-object？

//最新留言展示前面
$sql = "SELECT * FROM 'liuyan1' ORDER BY 'liuyan1'.'id' DESC";
$result = $conn->query($sql);

if($result->num_rows > 0){
    //输出数据
    while($row = $result->fetch_assoc()){
        ?>

请问显示：Notice: Trying to get property 'num_rows' of non-object in 是为什么啊