哪位高人可以指点一下。

Question

if (isset($_POST['id'])) {

    $id = $_POST['id'];

    $name = $_POST['name'];

    $num = $_POST['num'];

    $price = $_POST['price'];

  } 

  mysql_select_db('price');   

  $sql ="UPDATE webprice SET name='".$_POST['name']."', num=".$_POST['num'].",price=".$_POST['price']." WHERE id=".$_POST['id'];

  $sel = mysql_query($sql);

  $result = mysql_fetch_assoc($sel);

  if($result){

    header("Location: index.php");

  }else{

     echo "<p>更新失败。</p>";

  }

/错误提示：Warning: mysql_fetch_assoc() expects parameter 1 to be resource, boolean given in D:\phpStudy\WWW\m3\update.php on line 14 更新失败。