รบกวนช่วยทำ ระบบเช็คล็อกอิน ให้หน่อยครับ แบบดึงข้อมูล 2 ตาราง คือผมไม่เข้าใจอะครับ

<?php 
session_start(); 
mysql_connect("localhost","root",""); 
mysql_select_db("titan"); 
mysql_query('SET NAME UTF-8');
$xuser = mysql_real_escape_string($_POST['username']);
$xpswl = mysql_real_escape_string($_POST['password']);
$SQL = "SELECT * 
FROM `customers`,`employees` 
WHERE Cus_User = '$xuser'  and Cus_Pass = '$xpsw' 
and Emp_User= '$xuser' and Emp_Pass= '$xpsw' " ; 
$Query = mysql_query($SQL);
if( mysql_num_rows($Query)){ 
	$Result = mysql_fetch_assoc($Query);
	
	$_SESSION["Cus_Id"] = $Result["Cus_Id"]; 
	$_SESSION["Sit_Id"] = $Result["Sit_Id"]; 
	session_write_close(); 
	if($Result["Sit_Id"] == "001") { 
		header("location:adminpage.php"); 
	}else { 
		header("location:userpage.php"); 
	} 
}else{ 
	echo "Username and Password Incorrect!"; 
} 

CREATE TABLE `testing` (
  `code` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`code`)
) ENGINE=MyISAM AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;


INSERT INTO `testing` VALUES (1,'สก๊อตเทป \\\"ยูนิเทป\\\"'),(2,'สก๊อตเทปใส'),(3,'ทดสอบ \"ข้อมูล\"');

$SQL = "
SELECT * FROM `customers` WHERE Cus_User = '$xuser'  and Cus_Pass = '$xpsw' 
union all
SELECT * FROM `employees` WHERE Emp_User= '$xuser' and Emp_Pass= '$xpsw' " ;