Zipcode只属于一个

Question

我有这些表

create table Town (
   Town varchar(50)     not null,
   Province varchar(50) not null,
 constraint PK_Town primary key (Town)
 )

create table Streetpart (
    Zipcode varchar(6)   not null,
    Town varchar(50)     not null,
    Street varchar(50)   not null,
 constraint PK_Streetpart primary key (ZipCode),
 constraint FK_Streetpart foreign key (Town) references TOWN(Town)
)

create table ADRES (
      Zipcode      varchar(6) not null,
      Housenumeber int        not null,
 constraint PK_ADRES primary key (Zipcode, Housenumber),
 constraint FK_ADRES foreign key (Zipcode) references Streetpart(Zipcode)
)

我怎样才能确定只有 Zipcode 属于 Street ， Housenumber 和 Town 。

For example:
  Street         Housenumber   Zipcode Town
  Port du Louvre 12            1234AB  Paris
  Port du Louvre 12            5432PQ  Paris

是不允许的。

thanx求助。