To illustrate it better here is a sample data:

Column1 Column2
R1 NULL
R2 NULL
R2 X1
R2 X2
R3 NULL


As you can see the combined columns is unique, that is the requirement, is there a workaround that I can make a Composite Key out of those two tables?

Another thing, those two columns are PK from other tables, with the first column there is no problem cascading updates, but not so with the 2nd column, aside from a trigger is there any other trick that will let me cascade update the other tables using the reference from the 2nd column?

TIA