@celko - Thank you for your response and vluable suggestions. I don't see CONVALESCE function either. I am not familiar with ISO-11179 data element conventions. I just gave an example data set for the problem I am working on. The column names I chose are just to understand the scnario and it doesn't mean that I don't know any basics of data modeling, RDBMS or Date fields in SQL. I believe this forum is not just for experts and it is designed to help all levels of people.