Check constraint/foreign key combination

Question

I would like to add a constraint (check, FK, or otherwise) that forces a value to be either -1 OR be in a foreign table.  The statement below is invalid because you can't use subqueries, but I think it's clear what I'm trying to do.  Can anyone
suggest a different approach?
Thanks!
ALTER TABLE cfg.ClientFieldThreshold ADD CONSTRAINT CK_cfg_ClientFieldThreshold_DBListID
CHECK (DBListID = -1 OR DBListID IN (SELECT ID FROM dbo.vwClientDatabases)
/* the subquery here isn't allowed */

Answer 1

3 ways to do this. 
1) use NULL instead of -1 as the value that does not have to be in the foreign table.  Then you just need a normal foreign key (because a foreign key column that contains NULL is not checked against the foreign table.  Sample code to do this
Create Table FooParent(ParentID int primary key);
Create Table FooChild(ChildId int primary key, ParentID int Null, Constraint FooChildFK Foreign Key(ParentID) References FooParent);
go
-- Can add row with NULL
Insert FooChild(ChildId, ParentID) Values (1, Null);
go
-- But cannot add row with any other value that is not in FooParent
Insert FooChild(ChildId, ParentID) Values (2, 25);
go
-- Check result
Select * From FooChild;
go
Drop Table FooChild;
go
Drop Table FooParent;
2) Add a dummy row to the parent table with a primary key of -1.  Then, of course, all you need is a foreign key constraint.
3) Add a persisted computed column that is NULL whenever your column that you want to enforce the constraint against is equal to -1, otherwise just the value in that column.  Then create a foreign key on the computed column.  Sample code
Create Table FooParent(ParentID int primary key);
Create Table FooChild(ChildId int primary key, ParentID int Not Null, AlteredParentID As NULLIF(ParentID, -1) Persisted, Constraint FooChildFK Foreign Key(AlteredParentID) References FooParent);
go
-- Can add row with -1
Insert FooChild(ChildId, ParentID) Values (1, -1);
go
-- But cannot add row with any other value that is not in FooParent
Insert FooChild(ChildId, ParentID) Values (2, 25);
go
-- Check result
Select * From FooChild;
go
Drop Table FooChild;
go
Drop Table FooParent;
Tom