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A unidirectional many-to-one association is the most common kind of unidirectional association.

<class name="Person"><id name="id" column="personId"><generator class="native"/></id><many-to-one name="address"column="person_address_id"not-null="true"/></class><class name="Address"><id name="id" column="addressId"><generator class="native"/></id></class>

create table Person ( 

    personId bigint not null primary key, 

    person_address_id bigint not null,

    CONSTRAINT fk_person_address FOREIGN KEY (persion_address_id) REFERENCES address (addressId)
)

A unidirectional one-to-one association on a foreign key is almost identical. The only difference is the column unique constraint.

<class name="Person"><id name="id" column="personId"><generator class="native"/></id><many-to-one name="address"column="addressId"unique="true"not-null="true"/></class><class name="Address"><id name="id" column="addressId"><generator class="native"/></id></class>

create table Person ( personId bigint not null primary key, addressId bigint not null unique )create table Address ( addressId bigint not null primary key )

A unidirectional one-to-one association on a primary key usually uses a special id generator. (Notice that we've reversed the direction of the association in this example.)

<class name="Person"><id name="id" column="personId"><generator class="native"/></id></class><class name="Address"><id name="id" column="personId"><generator class="foreign"><param name="property">person</param></generator></id><one-to-one name="person" constrained="true"/></class>

create table Person ( personId bigint not null primary key )create table Address ( personId bigint not null primary key )

A unidirectional one-to-many association on a foreign key is a very unusual case, and is not really recommended.

<class name="Person"><id name="id" column="personId"><generator class="native"/></id><set name="addresses"><key column="address_person_id"not-null="true"/><one-to-many class="Address"/></set></class><class name="Address"><id name="id" column="addressId"><generator class="native"/></id></class>

create table Person ( personId bigint not null primary key )
create table Address ( addressId bigint not null primary key, address_person_id bigint not null )
alter table Address address_persion_id ADD CONSTRAINT fk_address_person FOREIGN KEY(address_person_id)
REFERENCES person(personId);

We think it's better to use a join table for this kind of association.