CREATE TABLE Person (

-- maybe Person is a Girl and maybe Girl is going out with Boy and Boy is a kind of Person and Person has Person ID,
GirlBoyfriendID                         int NULL,
-- Person is called Name,
Name                                    varchar NOT NULL,
-- Person has Person ID,
PersonID                                int IDENTITY NOT NULL,
PRIMARY KEY(PersonID),
FOREIGN KEY (GirlBoyfriendID) REFERENCES Person (PersonID)

) GO

CREATE VIEW dbo.GirlInPerson_BoyfriendID (GirlBoyfriendID) WITH SCHEMABINDING AS

SELECT GirlBoyfriendID FROM dbo.Person
WHERE   GirlBoyfriendID IS NOT NULL

GO

CREATE UNIQUE CLUSTERED INDEX IX_GirlInPersonByGirlBoyfriendID ON dbo.GirlInPerson_BoyfriendID(GirlBoyfriendID) GO