About foreign key

[raj malhotra]

Hi
In Head First SQL,on page 304 its written that

Foreign key don't have to be unique -infact they often aren't

on page 305 it says

You can use a foreign key to reference a unique value in the parent table.It doesn't have to be the primary key of the parent table,but it must be unique.

Somebody please clarify about it.

Thanks
Raj

[Campbell Ritchie]

create table address
(
  id longint unsigned auto_increment unique primary key,
  street_and_number varchar(40),
  town varchar(20)
);
 
create table person
(
  first_name varchar(30),
  last_name varchar(30),
  date_of_birth date,
  address_id longint unsigned
);
alter table person add constraint person_address_fk foreign key (address_id)
   references address(id)
   on update cascade on delete restrict;

You have one primary key for each address in the address table.
Several people in the person table can live at the same address, using the same value for their foreign key.

[Campbell Ritchie]

In my example you could not use town as a foreign key; there are likely to be several addresses for the same town, so you could not retrieve anybody's address.