selecting just one row

Does anyone no how to select one row from a table based on the first or part of an or expression ? Example: select price from oplist where product_code = ? and price_list = ? and (customer_code = ? or customer_code is null') I need to select one row if customer_code is not null or one row if customer_code is null. Not two rows as is happening due to one row with a customer_code and another with a null value. Does anyone have any suggestions? Regards, Peter Neville.