sql integrity 1-1 or 1-0 - sql

Question

I have a question on an exam that i don't know how to solve.
this is the question:
How do you keep the integrity of a 1-1 or 1-0 relationship between 2 tables?
Now a primary key and foreign key wouldn't work because then you can have a 1-m relationship. Maybe you can use unique in some way or alternative key?
We need to give the create table expressions and explain why this would work.

Answer 1

Here's an example of a 1-1 relationship in PostgreSQL. It requires the standard SQL feature of deferrable constraints:
create table employee (
id int primary key not null,
name varchar(20),
desk_id int not null,
constraint uq1 unique (desk_id),
constraint fk1 foreign key (desk_id) references desk (id)
deferrable initially immediate
);
create table desk (
id int primary key not null,
desk_code varchar(20).
employee_id int not null,
constraint uq2 unique (employee_id),
constraint fk2 foreign key (employee_id) references employee (id)
deferrable initially immediate
);
In simple English: every row of employee points to a desk. Every desk points to an employee.
Deferrable constraints are only implemented in PostgreSQL and Oracle, as far as I know.

Answer 2

I am going to give example from oracle
Here is 1:1 relationship example
create table state
(
sid number(3) ,
name varchar2(100),
gid number(3) not null,
constraint state_pk primary key (sid),
constraint gov_state_fk foreign key (gid) references gov(gid)
);
Here is 1:0..1 relationship (a.k.a. Surjection)
create table state
(
sid number(3) ,
name varchar2(100),
gid number(3) ,
constraint state_pk primary key (sid),
constraint gov_state_fk foreign key (gid) references gov(gid)
);
basicly what differs is,
In 1:0 relationship gid can be nullable
In 1:0..1 relationship gid cannot be null
EDIT: I also added Unicity for preserving 1:0..1 and 1:1 relationship.