Database table and Unique key

Hello All,

I have a database (customizing) table with two columns; one of them is a primary key and the other column should have the following behavior: Entering data for this column is optional. However, if some data is entered, it should be unique when compared to the existing values in that column. I tried unique index on this column but the problem is that multiple rows cannot have this column left unfilled. So, can anybody tell me how I can achieve this?

Regards,

Santhosh N