Unable to use inheritence in Hibernate

Please have a look at the following

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
  <class name="com.att_marks.students.Attendance_Btech">
  <id name="id" type="integer" column="id" >
   <generator class="increment"/>
  </id>

  <property name="year">
     <column name="Year" />
  </property>
  <property name="semister">
    <column name="Semister"/>
  </property>
  <property name="section">
    <column name="Section"/>
  </property>
  <property name="period">
    <column name="Period"/>
  </property>
  <property name="subjectCode">
    <column name="Subject_Code"/>
  </property>
  <property name="date">
    <column name="Date"/>
  </property>开发者_C百科;
  <property name="status">
    <column name="Status"/>
  </property>
  <union-subclass name="com.att_marks.students.Attendance_Btech_ECE" table="attendance_btech_ece">

  </union-subclass>
 </class>
</hibernate-mapping>

I have tables named attendance_btech_ece, attendance_btech_cse, attendance_btech_it, and so on. All these tables use the same schema as mentioned above for the Attendance_Btech class. I thought this should work but it isn't.

If the Attendance_Btech_ECE is subclass of Attendance_Btech then should work.