7.3.3. One-to-one

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <join table="PersonAddress" 
        optional="true">
        <key column="personId" 
            unique="true"/>
        <many-to-one name="address"
            column="addressId" 
            not-null="true"
            unique="true"/>
    </join>
</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 PersonAddress ( personId bigint not null primary key, addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )