How to implement a 'default' flag that can only be set on a single row For example, with a table similar to this:

create table foo(bar int identity, chk char(1) check (chk in('Y', 'N')));


It doesn't matter if the flag is implemented as a char(1), a bit or whatever. I just want to be able to enforce the constraint that it can only be set on a single row.
SQL Server 2008 - Filtered unique index

CREATE UNIQUE INDEX IX_Foo_chk ON dbo.Foo(chk) WHERE chk = 'Y'