hibernate的查询方式

hibernate的查询方式常见的主要分为三种: HQL, QBC（Query By Criteria）查询, 以及使用原生SQL查询

HQL查询

• HQL（Hibernate Query Language）提供了丰富灵活的查询方式，使用HQL进行查询也是Hibernate官方推荐使用的查询方式。

• HQL在语法结构上和SQL语句十分的相同，所以可以很快的上手进行使用。使用HQL需要用到Hibernate中的Query对象，该对象专门执行HQL方式的操作。

查询所有示例
session.beginTransaction();
String hql = "from User"; // from 后跟的是要查询的对象，而不是表
Query query = session.createQuery(hql);
List<User> userList = query.list();
for(User user:userList){
  System.out.println(user.getUserName());
}
session.getTransaction().commit();

获取一个不完整的对象
session.beginTransaction();
String hql = "select userName from User";
Query query = session.createQuery(hql);
List nameList = query.list();
for(Object obj:nameList){
  System.out.println(obj);
}
session.getTransaction().commit();

// 多个属性的话，需要用object[]接收

session.beginTransaction();
String hql = "select userName,userPwd from User";
Query query = session.createQuery(hql);
List nameList = query.list(); 

for(Object obj:nameList){
    Object[] array = (Object[]) obj; // 转成object[]
    System.out.println("name:" + array[0]);
    System.out.println("pwd:" + array[1]);
}
session.getTransaction().commit();

统计和分组查询
session.beginTransaction();
String hql = "select count(*),max(id) from User";
Query query = session.createQuery(hql);
List nameList = query.list();
for(Object obj:nameList){
    Object[] array = (Object[]) obj;
    System.out.println("count:" + array[0]);
    System.out.println("max:" + array[1]);
}
session.getTransaction().commit();

更多写法
select distinct name from Student; 

select max(age) from Student; 

select count(age),age from Student group by age; 

from Student order by age;

HQL占位符和分页

session.beginTransaction();
        String sql ="select uname,password from User where enable=?";
        Query query =session.createQuery(sql);
        //占位符内容
        query.setBoolean(0, true);
        //第几个开始
        query.setFirstResult(0);
        //显示个数
        query.setMaxResults(2);
        List<Object[]> list = query.list();
        for (Object[] user : list) {
            System.out.println(user[0]+"--"+user[1]);
        }
        session.getTransaction().commit();

原生SQL查询

addEntity()示例
session.beginTransaction(); 

String sql = "select id,username,userpwd from t_user"; 

// addEntity()可以告诉Hibernate你想要封装成对象的类型，然后自动为你封装
SQLQuery query = session.createSQLQuery(sql).addEntity(User.class); 

List<User> list = query.list(); 

for(User user : list){
  System.out.println(user.getUserName());
} 

session.getTransaction().commit();

uniqueResult示例
session.beginTransaction(); 

String sql = "select id,username,userpwd from t_user where id = 2";
SQLQuery query = session.createSQLQuery(sql).addEntity(User.class); 

User user = (User) query.uniqueResult(); 

System.out.println(user.getUserName()); 

session.getTransaction().commit();

hibernate的查询方式