Temporal Uniqueness

CREATE FUNCTION [dbo].[pAC_NAM_Actor_Name] (@changingTimepoint date)
RETURNS TABLE WITH SCHEMABINDING AS RETURN
SELECT
   AC_ID,
   AC_NAM_Actor_Name,
   AC_NAM_ChangedAt
FROM
   [dbo].[rAC_NAM_Actor_Name](@changingTimepoint) AC_NAM
WHERE
   AC_NAM_ChangedAt = (
        SELECT
            MAX(sub.AC_NAM_ChangedAt)
        FROM
            [dbo].[rAC_NAM_Actor_Name](@changingTimepoint) sub
        WHERE
            sub.AC_ID = AC_NAM.AC_ID
   );

GO
CREATE FUNCTION [dbo].[qAC_NAM_Actor_Name](
    @AC_ID int,
    @AC_NAM_Actor_Name varchar(42),
    @AC_NAM_ChangedAt datetime
)
RETURNS INT
BEGIN
RETURN (
    SELECT
        COUNT(*)
    FROM
        [dbo].[pAC_NAM_Actor_Name](@AC_NAM_ChangedAt)
    WHERE
        AC_ID <> @AC_ID
    AND
        AC_NAM_Actor_Name = @AC_NAM_Actor_Name
)
END

GO
ALTER TABLE [dbo].[AC_NAM_Actor_Name]
ADD CONSTRAINT uqAC_NAM_Actor_Name
CHECK ([dbo].[qAC_NAM_Actor_Name](AC_ID, AC_NAM_Actor_Name, AC_NAM_ChangedAt) = 0);

GO
INSERT INTO lAC_Actor (AC_NAM_Actor_Name, AC_NAM_ChangedAt)
VALUES('Nathalie', '2009-09-21');
INSERT INTO lAC_Actor (AC_ID, AC_NAM_Actor_Name, AC_NAM_ChangedAt)
VALUES(1, 'Nat', '2012-06-12');
INSERT INTO lAC_Actor (AC_NAM_Actor_Name, AC_NAM_ChangedAt)
VALUES('Nathalie', '2012-06-12');
INSERT INTO lAC_Actor (AC_NAM_Actor_Name, AC_NAM_ChangedAt)
VALUES('Nathalie', '2012-06-12');
-- next insert should fail because of the constraint
INSERT INTO lAC_Actor (AC_NAM_Actor_Name, AC_NAM_ChangedAt)
VALUES('Nathalie', '2012-12-12');

SELECT * FROM lAC_Actor;