Hibernate继承映射总结

Hibernate继承映射三种策略中:

每个类分层结构是一张表:

??? 表现在数据库中父类与子类在同一张表中,这样查询起来方便.

每个子类是一张表:

?? 表现在数据库中父类与子类分别在不同的表中,但子类只有子类定义的属性所对应的字段,这样做查询的时候会用到inner join或者是left join等.

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最后一种是每个具体类是一张表:

?? 表现在数据库中也是父类与子类分别在不同的表中,不过子类也包括了父类的所有字段.这样如果直接从子类load就比较快,而如果从父类load的话sql就会比较麻烦.

比如第三种策略,如果我只load pig对象:

Pig p = (Pig)session.load(Pig.class, 1);

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则发出sql如下:

Hibernate:     /* load com.lwf.hibernate.extend1.Pig */ select        pig0_.id as id0_0_,        pig0_.name as name0_0_,        pig0_.sex as sex0_0_,        pig0_.weight as weight1_0_     from        t_pig pig0_     where        pig0_.id=?

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而如果我load animal对象:

Animal a = (Animal)session.load(Animal.class, 1);

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则发出sql如下:

Hibernate:     /* load com.lwf.hibernate.extend1.Animal */ select        animal0_.id as id0_0_,        animal0_.name as name0_0_,        animal0_.sex as sex0_0_,        animal0_.weight as weight1_0_,        animal0_.height as height2_0_,        animal0_.clazz_ as clazz_0_     from        ( select            id,            name,            sex,            weight,            null as height,            1 as clazz_         from            t_pig         union        select            id,            name,            sex,            null as weight,            height,            2 as clazz_         from            t_bird     ) animal0_ where    animal0_.id=?

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看看就知道谁要方便...