22-Hibernate检索方式

1）HQL：

　　HQL是Hibernate查询语言的简称，是一种面向对象的查询语言，使用类，对象，和属性，没有表和字段的概念。

使用HQL查询的基本步骤：获得session ----》编写HQL语句 ------》创建Query对象 -------》执行查询，获得结果。

        String hql = "from Employee";
        Query query = session.createQuery(hql);
        List<Employee> list = query.list();

        Iterator itor = list.iterator();
        while(itor.hasNext()){
            Employee e = (Employee) itor.next();
            System.out.println(e.getName());
        }

对查询结果排序 String hql = "from Employee as e order by e.age desc";

查询部分属性

        String hql = "select e.name, e.age from Employee as e";
        Query query = session.createQuery(hql);
        List<Employee> list = query.list();

        Iterator itor = list.iterator();
        while(itor.hasNext()){
            Object[] obj = (Object[])itor.next();
            System.out.println(obj[0] + "--" + obj[1]);
        }

HQL聚集函数：count(), min(), max(), sum(), avg()

        String hql = "select count(e) from Employee as e";
        Query query = session.createQuery(hql);
        Long count = (Long)query.uniqueResult();

分组查询 String hql = "select a.employee.name, count(*) from Address a group by a.employee";

动态实例查询：在属性查询中返回的是对象数组，不易操作，为了提高检索效率，可将检索出的属性分装到一个实体类。需要提供相应构造方法。

        String hql = "select new Address(a.id,a.shortName) from Address a";
        Query query = session.createQuery(hql);
        List list = query.list();
        Iterator itor = list.iterator();
        while(itor.hasNext()){
            Address a = (Address)itor.next();
            System.out.println(a.getShortName());
        }

分页查询

        //从第一个对象开始查询
        query.setFirstResult(0);
        //返回三个对象
        query.setMaxResults(3);

条件查询

按参数位置

        String hql = "from Employee e where e.name like ?";
        Query query = session.createQuery(hql);
        query.setString(0, "dudu");

按参数名

        String hql = "from Employee e where e.name = :ename";
        Query query = session.createQuery(hql);
        query.setParameter("ename","dudu");

连接查询
- 内连接inner join String hql = "select a from Address a inner join a.employee e where e.name = 'dudu'";
- 隐式内连接(在HQL语句中看不到join，但是实际上已经发生内连接) String hql = "select a from Address a, Employee e where a.employee = e and e.name = 'dudu'";
- 左外连接 left join
- 右外连接right join
- 交叉连接：HQL中的内连接和外连接主要为关联类连接查询，对于相互之间毫无关系的对象，可以使用交叉连接进行查询。 String hql = "select Person p, User u where p.id = u.id";

相关子查询：子查询使用外层查询中的对象名。e.address为set类型。
```
String hql = "select Employee e where (select count(*) from e.address) > 1";
```

无关子查询：子查询与外层查询语句无关。

        String hql = "select Book b where b.unitPrice < (select avg(b1.unitPrice) from Book b1)";

2）QBC:

Order By Criteria 。Criteria是Hibernate API 提供的一个查询接口，又称为对象查询。

3) SQL: