-- This defines the ID space shared by different derived table types.
CREATE TABLE GenericObjects
(
   ID      INTEGER NOT NULL AUTO_INCREMENT,

   CONSTRAINT PK_GenericObjects PRIMARY KEY (ID)
);

-- There will be other similar tables dervied from GenericObjects.
CREATE TABLE DerivedObjects
(
   ID      INTEGER NOT NULL,

   OtherData   INTEGER NULL,

   PRIMARY KEY PK_DerivedObjects(ID),
   FOREIGN KEY (ID) REFERENCES GenericObjects (ID) ON DELETE CASCADE
);

<hibernate-mapping>
    <class name="GenericObject" table="GenericObjects">
        <id name="id" column="ID">
            <generator class="native"/>
        </id>
    </class>
</hibernate-mapping>

<hibernate-mapping>
    <joined-subclass
             name="DerivedObject"
             table="DerivedObjects"
             extends="GenericObject">
        <key column="ID"/>

        <property name="otherData" column="OtherData"/>
    </joined-subclass>
</hibernate-mapping>