PHP MySQL问题

Denis

以下代码为什么会出现下面的错误信息?
<?php
include "include/config.inc";
$id = mysql_connect($servername,$dbusername,$dbpassword);
mysql_select_db($dbnam,$id);
$result = mysql_query("select CategoryID,CategoryName from Categories",$id);
echo mysql_num_rows($result);
while($datas = mysql_fetch_row($result))
  echo $datas[1][0];
?>

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in C:\XAMP\htdocs\MIT\test.php on line 6

Warning: mysql_fetch_row(): supplied argument is not a valid MySQL result resource in C:\XAMP\htdocs\MIT\test.php on line 7