ช่วยทีค่ะ error Warning: mysql_num_rows(): supplied argument is not a valid

<?php 
	session_start(); 		//เริ่ม session 
	$username=$_POST[username];
	$password=$_POST[password];
	
	if(isset($HTTP_POST_VARS['Submit'])=='Login'){	
		if($username==""||$password==""){
					echo "<script>";
					echo "alert('Login Fail');";
					echo "window.history.go(-1);";
					echo "</script>";	
		}else{
		include("../connections/connect.php");
 // ตรวจสอบ status
 		$sql="SELECT * FROM tb_users WHERE username='$username' AND password='$password'";
		$result=mysql_query($sql);	
		$num=mysql_num_rows($result);
		$arr=mysql_fetch_array($result);
				$user_ID=$arr[user_ID];
				$status=$arr[stutus];
		 // กำหนดให้ session 
				session_register("status");
				session_register("user_ID");
				session_register("firstname");
				session_register("lastname");
				
		if($num>0){
			if($status=="admin"){
				header("location:admin/index_admin.php?user_ID=$user_ID");
			}elseif($status=="clients"){
				// หาชื่อและนามสกุล ตามเงื่อนไขข้างบน
			$sql_2 ="SELECT firstname,lastname FROM tb_clients WHERE user_ID='$user_ID' ";	
			$res=mysql_query($sql_2)or die(mysql_error());
			$row=mysql_fetch_array($res);
				$firstname= $row[firstname];
				$lastname= $row[lastname];
				header("location:index_clients.php?user_ID=$user_ID");
			}else{
				echo "<script>";
				echo "alert('Login Fail');";
				echo "window.history.go(-1);";
				echo "</script>";
			}	
		}
	}
}
mysql_close();
?>

<?php
	$host="localhost";
	$user="root";
	$pw="1234";
	$dbname="db_dokkaew";
	$connect=mysql_connect($host,$user,$pw);
	if(!$connect){
		echo "<h3>ERROR : ไม่สามารถติดต่อฐานข้อมูลได้</h3>";
	}else{
			mysql_select_db($dbname,$connect);
			mysql_query("SET NAMES TIS620",$connect);
		}
?>