I think the problem is somehow related to roman numeral conversion when some values are null. The following seems to work: select case when gen is not null then to_char(gen,'rn') else null end, case when gen is not null then to_char(gen+1,'rn') else null end from cname; While the following doesn't: select to_char(gen,'rn'), to_char(gen+1,'rn') cname;
