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) should be unique, BUT only if the row is not deleted (isDeleted=0).

Is it possible to define such a constraint in SQLServer 2008? (without using triggers)

P.S. Setting (FieldID, FormID, isDeleted) to be unique adds the possibility to mark one row as deleted, but i would like to have the chance to set n rows (per FieldID,FormID) to isDeleted = 1, and to have only one with isDeleted = 0

Answer 1

You can have a unique index, using the SQL Server 2008 filtered indexes feature, or you can apply a UNIQUE index against a view (poor man's filtered index, works against earlier versions), but you cannot have a UNIQUE constraint such as you've described.

An example of the filtered index:

 CREATE UNIQUE NONCLUSTERED INDEX IX_FieldsOnForms_NonDeletedUnique ON FieldsOnForms (FieldID,FormID) WHERE isDeleted=0

Answer 2

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. :)