Rule Definition
If you require polymorphic associations or queries, and subclasses declare many properties (subclasses differ mainly by the data they hold), lean toward the table-per-subclass approach. Note that By default, choose table-per-class-hierarchy for simple problems. For more complex cases (or when you're overruled by a data modeler insisting upon the importance of nullability constraints), you should consider the table-per-subclass strategy. But at that point, ask yourself whether it might be better to remodel inheritance as delegation in the object model. Complex inheritance is often best avoided for all sorts of reasons unrelated to persistence or ORM.

Remediation
Use the table-per-subclass strategy to map the hierarchy.

Violation Code Sample

----> animal.hbm.xml

<class name="Animal" table="ANIMAL">
    <id name="id" type="long" column="ANIMAL_ID">
        <generator class="native"/>
    </id>
    <discriminator column="TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>

    // VIOLATION: one of the subclass attributes count 
    // is greater or equal to 3 and table-per-subclass
    // strategy is not used

    <subclass name="Dog" discriminator-value="DOG">
        <property name="race" column="RACE"/>
    </subclass>

    <subclass name="Cat" discriminator-value="CAT">
        <property name="color" column="COLOR"/>
        <property name="country" column="COUNTRY"/>
        <property name="age" column="AGE"/>
        <property name="owner" column="OWNER">
    </subclass>
</class>

Fixed Code Sample

<class name="Animal" table="ANIMAL">
    <id name="id" type="long" column="ANIMAL_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="AMOUNT"/>

    // FIXED table-per-subclass strategy is used
    <joined-subclass name="Dog" table="DOG">
        <key column="DOG_ID"/>
        <property name="race" column="RACE"/>
    </joined-subclass>

    <joined-subclass name="Cat" table="CAT">
        <key column="CAT_ID"/>
        <property name="color" column="COLOR"/>
        <property name="country" column="COUNTRY"/>
        <property name="age" column="AGE"/>
        <property name="owner" column="OWNER">
    </joined-subclass>
</class>

http://www.developer.com/open/print.php/10930_3559931_5 Hibernate in Action (ISBN 1932394-15-X) p 105