NULLs in primary keys

My customer has a table of services charges, keyed on the type of service and the cost centre. Some services have different charges for each cost centre, others are global. At present the global ones have a cost centre of NULL. I have a stored procedure containing a query something like: SELECT things FROM charges_table WHERE service_type = p_service_type AND cost_centre = p_cost_centre If the row we're after has a NULL cost_centre and therefore p_cost_centre has been set NULL, the row is never found. The client has OnLine 5.00. Now I can understand why nulls in a primary key are generally not a very nice idea, but this must be a common enough business requirement. Questions: does anyone know off the top of their heads whether this would work in 4GL (which they don't have), ESQL/C or a later version of SPL? Has anyone had experience of getting around a similar problem? (I've tried getting them to use blank instead of null in the interim and this works but I don't like it) Thanks in anticipation. akent@cix.compulink.co.uk (Andy Kent) -------------------------------------