HQL多表查询

Posted 2020-10-01

1. 多表的查询进来使用HQL语句进行查询，HQL语句和SQL语句的查询语法比较类似。
    * 内连接查询
        * 显示内连接
            * select * from customers c inner join orders o on c.cid = o.cno;
        * 隐式内连接
            * select * from customers c,orders o where c.cid = o.cno;
    * 外连接查询
        * 左外连接
            * select * from customers c left join orders o on c.cid = o.cno;
        * 右外连接
            * select * from customers c right join orders o on c.cid = o.cno;

2. HQL的多表查询
    * 迫切和非迫切：
        * 非迫切返回结果是Object[]
        * 迫切连接返回的结果是对象，把客户的信息封装到客户的对象中，把订单的信息封装到客户的Set集合中。
Set集合解决重复问题

/**
 * HQL多表查询
 * @author bamaw
 *
 */
public class Demo5 {
    
    /**
     * 查询客户，客户和联系人有关联
     * select * from cst_customer c,cst_linkman l where c.id = l.id;
     * 有两个问题：1.数据的重复问题  2.数据没有封装到对象当中
     */
    @Test
    public void run1() {
        Session session = HibernateUtils.getCurrentSession();
        Transaction tx = session.beginTransaction();
        //内连接查询
        Query query = session.createQuery("from Customer c inner join c.linkmans");
        //返回的是数组
        List<Object[]> list = query.list();
        for (Object[] objects : list) {
            System.out.println(Arrays.toString(objects));
        }
    }
    
    /**
     * 将数据封装到对象中
     * 提供关键字:使用fetch 迫切连接
     */
    @Test
    public void run2() {
        Session session = HibernateUtils.getCurrentSession();
        Transaction tx = session.beginTransaction();
        //内连接查询
        Query query = session.createQuery("from Customer c inner join fetch c.linkmans");
        //返回的是对象
        List<Customer> list = query.list();
        for (Customer customer : list) {
            System.out.println(customer);
        }
    }
    
    /**
     * 解决重复问题
     */
    @Test
    public void run3() {
        Session session = HibernateUtils.getCurrentSession();
        Transaction tx = session.beginTransaction();
        //内连接
        Query query = session.createQuery("from Customer c inner join fetch c.linkmans");
        //返回对象
        List<Customer> list = query.list();
        //解决重复问题，编程中都是用set集合来使用 --因为不可重复
        Set<Customer> set = new HashSet<Customer>(list);
        for (Customer customer : set) {
            System.out.println(customer);
        }
    }
    
    /**
     * 左外连接查询
     */
    @Test
    public void run4() {
        Session session = HibernateUtils.getCurrentSession();
        Transaction tx = session.beginTransaction();
        Query query = session.createQuery("from Customer c left join fetch c.linkmans");
        List<Customer> list = query.list();
        //解决重复问题
        Set<Customer> set = new HashSet<Customer>(list);
        for (Customer customer : set) {
            System.out.println(customer);
        }
    }
}