Factoring out nulls in bill-of-materials style relations

PERSON { A, B }
PERSON { B, C }
PERSON { C, NULL }

PERSON { A, B }
PERSON { B, C }
PERSON { C, NULL }

PERSON { A, B }
PERSON { B, C }
PERSON { C, NULL }

PERSON { A, FEMALE, B }
PERSON { B, MALE, NULL }
PERSON { C, FEMALE, NULL }

CREATE TABLE People
(
     person_name     VARCHAR(100),
     CONSTRAINT PK_People PRIMARY KEY (person_name)
)
GO
CREATE TABLE Spouses
(
     person_name     VARCHAR(100),
     spouse_name     VARCHAR(100),
     CONSTRAINT PK_Spouses PRIMARY KEY (person_name),
     CONSTRAINT FK_Spouses_People FOREIGN KEY (person_name) REFERENCES People (person_name)
)
GO