if not exists(SELECT  1   as Tag, NULL as Parent,
         Customers.CustomerID as [Customer! 1 !CustomerID],
         NULL as [Order! 2 !OrderID]
FROM Customers where  1  =  0 )
SELECT  1                     as Tag, 
         NULL                 as Parent,
         NULL                 as [Customer! 1 !CustomerID],
         NULL                 as [Order! 2 !OrderID]
UNION ALL
SELECT  2 , 
          1 ,
         Customers.CustomerID,
         Orders.OrderID
FROM Customers, Orders
WHERE Customers.CustomerID = Orders.CustomerID
ORDER BY [Customer! 1 !CustomerID], [Order! 2 !OrderID]
FOR XML EXPLICIT
else
SELECT  1                     as Tag, 
         NULL                 as Parent,
         Customers.CustomerID as [Customer! 1 !CustomerID],
         NULL                 as [Order! 2 !OrderID]
FROM Customers where  1  =  0 
UNION ALL
SELECT  2 , 
          1 ,
         Customers.CustomerID,
         Orders.OrderID
FROM Customers, Orders
WHERE Customers.CustomerID = Orders.CustomerID
ORDER BY [Customer! 1 !CustomerID], [Order! 2 !OrderID]
FOR XML EXPLICIT