[jachguate]

creo que algo así debiera funcionar...

Select Cs.Customer_code Codigo, Cs.Customer_name Cliente, 
       Sum(Tr.Units) Unidades
  From Customer Cs
       Left Outer Join Transactions Tr 
       on Tr.customer_code = Cs.customer_code
 Where (tr.transaction_date is null) or
       (TR.Transaction_Date between "3/1/2003 00:00:00" and "3/31/2003 23:59:59")
 Group BY Cs.Customer_Code, Cs.Customer_Name
 Order by 3 DESC

Hasta luego.