ทำไมมัน Erorr ค่ะ กับคำสั่งนี้ $num = mysql_num_rows($result) ;

<?
/************* เอาไว้บนสุดของเพจ*********************/
session_start() ;

ob_start();

/**********************************************/

$user_login=$_POST[user_login] ;

$pass_login=$_POST[pass_login] ;

if ($user_login=="" or $pass_login=="") {
echo "<h3> ERROR : ระบุข้อมูลให้ครบ</h3>" ;
exit() ;
}

include "connect.php" ;

$sql="select * form tb_member where username='$user_login' and password='$pass_login'" ;

$result = mysql_db_query($dbname,$sql) ;

$num = mysql_num_rows($result) ;


if ($num<=0) {

echo "<h3>ERROR : Username หรือ Password ไม่ถูกต้อง</h3>" ;

} else {

$_SESSION[sess_userid]=session_id() ;

$_SESSION[sess_username]=$user_login ;

header("Location: main.php") ;

}

mysql_close()

?>

<?php

include "connect.php" ;

echo $sql="select * form tb_member where username='$user_login' and password='$pass_login' " ;
echo $result = mysql_db_query($dbname,$sql) ;
echo $num = mysql_num_rows($result) ;

?>

<?php
$result = mysql_db_query($dbname,$sql)  or die(mysql_error());

?>