hibernate 之joined-subclass 标签

<?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 package="com.randy.bean2">
  <class name="Item" table="t_Item">
    
    <id name="id">
    <generator class="native"/>
    </id>
    <property name="name" />
    <joined-subclass name="Book" table="t_book">
      <key column="sid" />
      <property name="page" />
    </joined-subclass>
    
    <joined-subclass name="Dvd" table="t_dvd">
      <key column="ids" />
      <property name="region" />
    </joined-subclass>
  </class>
    
    
</hibernate-mapping>

 

以上是 实体的映射文件

    String hql ="from Item";
    List userlist = session.createQuery(hql).list();

 

Hibernate 会生成如下的SQL

select item0_.id as id0_,
             item0_.name as name0_,
             item0_1_.page as page1_,
             item0_2_.region as region2_,
             case
                 when item0_1_.sid is not null then
                    1
                 when item0_2_.ids is not null then
                    2
                 when item0_.id is not null then
                    0
             end as clazz_
    from t_Item item0_
    left outer join t_book item0_1_ on item0_.id = item0_1_.sid
    left outer join t_dvd item0_2_ on item0_.id = item0_2_.ids

 

 

但是如果是 Table per Concrete Class ，（一个子类对应一个表，父类没有表）；

则会生成2条sql， 分别查询2个子表