UNIQUE constraint controlled by a bit column

Question

I have a table, something like

FieldsOnForms(
 FieldID int (FK_Fields)
 FormID int (FK_Forms)
 isDeleted bit
)

The pair (FieldID,FormID) s

Answer 1

You could change your IsDeleted column to a DeletedDate and make it a DATETIME with the exact time when the row was logically deleted. Alternatively, you could add a DeletedDate column and then create an IsDeleted computed column against that so that you still have that column available if it's being used in code. You would then of course put a unique index over the DeletedDate (in addition to the FieldID and FormId) instead of the IsDeleted column. That would allow exactly one NULL column.

Albin posted a solution similar to this, but then deleted it. I'm not sure why, but if he re-posts it then his was here before mine. :)