Sorry for my English. I´m not good writer.
Well, I made a Union
Make sure your select lists are same length and datatype.
SELECT CUSTOMER_REF_NAM AS CustRef, CASE WHEN cust_ref = 'X' THEN 'value' END FROM SaleOrder
SELECT CUSTOMER_REF_NAM AS CustRef , CASE WHEN cust_ref = 'X' THEN 'value' END FROM AL1;