hibernate笔记-常用查询方法

介绍hibernate常用查询方法

用到的表

CREATE TABLE `department` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8

CREATE TABLE `employee` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `first_name` varchar(255) DEFAULT NULL,
  `last_name` varchar(255) DEFAULT NULL,
  `salary` int(11) DEFAULT NULL,
  `department_id` int(11) DEFAULT NULL,
  `country_id` int(11) NOT NULL,
  `phone` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `FK75C8D6AED43F9198` (`department_id`),
  CONSTRAINT `FK75C8D6AED43F9198` FOREIGN KEY (`department_id`) REFERENCES `department` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8

1. 查询department所有内容

 String sql = "from Department t";
 Query q = session.createQuery(sql);
 List<Department> departments = q.list();

hibernate可以将查询结果自动映射到pojo中

2. 占位符的使用

String sql = "from Department t where t.name=:name";
Query qq = session.createQuery(sql);
List<Department> departments = qq.setParameter("name", "技术部").list();

或者

String sql = "from Department t where t.name=?";
Query qq = session.createQuery(sql);
List<Department> departments = qq.setParameter(0, "技术部").list();

3. 使用主键获取实体
   hibernate可以使用session.load()或者session.get()方法获取主键对应的实体.
   两者的区别是:使用load如果没有实体,抛出异常,使用get没有实体返回null

4. hql返回数组

String sql = "select t,(select count(*) from t.employees es where es.departmentId=t.id and es.salary>10) as cc  from Department t  group by t";
Query qq = session.createQuery(sql);
List<Object[]> list = qq.list();

5. hql返回map

String sql = "select new Map(t,(select count(*) from t.employees es where es.departmentId=t.id and es.salary>=10) as cc)  from Department t  group by t";
List<Map<String, Object>> map = session.createQuery(sql).list();

对于有别名的列直接使用别名获取
map.get("cc")
无别名的列则使用所在的位置获取
map.get("0")