MySQL中null值问题

<?php
session_start();
$db=mysqli_connect('localhost','root','root','qa');
$tname=$_POST['teacher'];
$q="select schedule,place from time where Tname='$tname' and Sname is null";
$arr=mysqli_query($db,$q);
echo "您选择了". $tname."老师"."<br/>请继续选择时刻："."<br/>";
?>
<table border="1" cellspacing="0">
    <tr>
        <th>时刻：</th>
        <th>地点：</th>
    </tr>

  <?php
    while ($result = mysqli_fetch_assoc($arr)) {
        ?>
        <tr>
            <td><?php echo $result['schedule']; ?></td>
            <td><?php echo $result['place']; ?></td>
        </tr>
        <?php
    }
 ?>
    </table>
 <form method="post" action="Stime1.php">
     您选择的时间是：<input type="datetime" name="time" />
     您的问题是：<textarea name="question"></textarea>
    <input type="submit" name="sure" value="确定"/>
</form>