HQL中的内连接、左连接、右连接

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内连接：把两面共有的部分查出来
左连接：把左边的部分全查出来
右连接：把右边的部分全查出来

/**
* inner join
*/
public void testquery1(){
    Session session = null;
    try{
        session = HibernateUtils2.singleInstance.getSession();
        session.beginTransaction();

        //可以省略inner，因为只要写了join，默认就是inner内连接
        //List lists = (List)session.createQuery("select c.name, s.name from Student s join s.classes c").list();

        //写上的话，就是这个样子啦
        List lists = (List)session.createQuery("select c.name, s.name from Student s inner join s.classes c").list();
        for (Iterator iter = lists.iterator();iter.hasNext();) {
            Object[] obj = (Object[])iter.next();
            System.out.println(obj[0] + "," + obj[1]);
        }
    }catch(Exception e){
        e.printStackTrace();
        session.getTransaction().rollback();
    }finally{
        HibernateUtils2.singleInstance.closeSession(session);
    }
}

/**
* left join
*/
public void testquery2(){
    Session session = null;
    try{
        session = HibernateUtils2.singleInstance.getSession();
        session.beginTransaction();

        //把没有班级的学生也查出来，注意left join 后接的表有点特殊，是c.student即班级中的student属性，hibernate会依照这个属性帮我们找到相应的表
        List lists = (List)session.createQuery("select c.name, s.name from Student s left join c.student s").list();
        for (Iterator iter = lists.iterator();iter.hasNext();) {
            Object[] obj = (Object[])iter.next();
            System.out.println(obj[0] + "," + obj[1]);
        }
    }catch(Exception e){
        e.printStackTrace();
        session.getTransaction().rollback();
    }finally{
        HibernateUtils2.singleInstance.closeSession(session);
    }
}

/**
* right join
*/
public void testquery2(){
    Session session = null;
    try{
        session = HibernateUtils2.singleInstance.getSession();
        session.beginTransaction();

        //把没有学生的班级也查出来，注意right join 后接的也比较特殊，理由同上
        List lists = (List)session.createQuery("select c.name, s.name from Student s right join c.student s").list();
        for (Iterator iter = lists.iterator();iter.hasNext();) {
            Object[] obj = (Object[])iter.next();
            System.out.println(obj[0] + "," + obj[1]);
        }
    }catch(Exception e){
        e.printStackTrace();
        session.getTransaction().rollback();
    }finally{
        HibernateUtils2.singleInstance.closeSession(session);
    }
}

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