hibernate>查询语言hql

HQL(Hibernate Query Language)

 

1、简单属性查询        2、实体对象查询      3、条件查询      4、使用sql进行查询    5、外置命名查询  6、查询过滤器
7、分页查询               8、对象导航查询      9、连接查询     10、统计查询              11、DML风格的操作

一、简单属性查询
 * 单一属性查询，返回结果集属性列表，元素类型和实体类中相应的属性类型一致
 * 多个属性查询,返回的集合元素是对象数组,数组元素的类型和对应的属性在实体类中的类型一致
   数组的长度取决与select中属性的个数
 * 如果认为返回数组不够对象化，可以采用hql动态实例化Student对象

  在hql中关键字不区分大小写，但是属性和类名区分大小写

SimplePropertyQueryTest.java 

package com.ncepu.hibernate;

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 简单属性查询
 * @author Administrator
 *
 */
public class SimplePropertyQueryTest extends TestCase {
	
	/**
	 * 单一属性查询
	 */
	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//返回结果集属性列表，元素类型和实体类中相应的属性类型一致
			List students = session.createQuery("select name from Student").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				String name = (String)iter.next();
				System.out.println(name);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}

	/**
	 * 多个属性查询
	 */
	public void testQuery2() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//查询多个属性，其集合元素是对象数组
			//数组元素的类型和对应的属性在实体类中的类型一致
			//数组的长度取决与select中属性的个数
			List students = session.createQuery("select id, name from Student").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}

	/**
	 * 返回Student实体对象
	 */
	public void testQuery3() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//如果认为返回数组不够对象化，可以采用hql动态实例化Student对象
			//此时list中为Student对象集合
			List students = session.createQuery("select new Student(id, name) from Student").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println(student.getId() + "," + student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}

	/**
	 * 使用别名
	 */
	public void testQuery4() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//可以使用别名
			List students = session.createQuery("select s.id, s.name from Student s").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}

	/**
	 * 使用别名
	 */
	public void testQuery5() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//可以使用as命名别名
			List students = session.createQuery("select s.id, s.name from Student as s").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}
	
}

二、实体对象查询

1、实体对象查询

//可以忽略select
   List students = session.createQuery("from Student").list();

//可以忽略select,表可以使用别名
   List students = session.createQuery("from Student s").list();

//可以忽略select,表可以使用as命名别名
   List students = session.createQuery("from Student as s").list();

//使用select查询实体对象，必须采用别名

List students = session.createQuery("select s from Student as s").list();

//不支持select * from .....这样的查询语句
   List students = session.createQuery("select * from Student").list();

2、N + 1问题

在默认情况下，使用query.iterate查询，有可以能出现N+1问题
所谓的N+1是在查询的时候发出了N+1条sql语句
   1: 首先发出一条查询对象id列表的sql
   N: 根据id列表到缓存中查询，如果缓存中不存在与之匹配的数据，那么会根据id发出相应的sql语句
3、list和iterate的区别？
  * list每次都会发出sql语句，list会向缓存中放入数据，而不利用缓存中的数据
  * iterate：在默认情况下iterate利用缓存数据，但如果缓存中不存在数据有可以能出现N+1问题
SimpleObjectQueryTest.java

package com.ncepu.hibernate;

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 实体对象查询
 * @author Administrator
 *
 */
public class SimpleObjectQueryTest2 extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			/**
			 * 采用list查询发出一条查询语句，取得Student对象数据、
			 * 
			 * Hibernate: select student0_.id as id1_, student0_.name as name1_, 
			 * student0_.createTime as createTime1_, student0_.classesid as classesid1_ 
			 * from t_student student0_
			 * 
			 */
			List students = session.createQuery("from Student").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}
	
	public void testQuery2() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			/**
			 * 出现N+1问题
			 * 
			 * 1：发出查询id列表的sql
			 *   Hibernate: select student0_.id as col_0_0_ from t_student student0_
			 * 
			 * N：在依次发出根据id查询Student对象的sql
			 * Hibernate: select student0_.id as id1_0_, student0_.name as name1_0_, 
			 * student0_.createTime as createTime1_0_, student0_.classesid as classesid1_0_ 
			 * from t_student student0_ where student0_.id=?
			 *  
			 */
			Iterator iter = session.createQuery("from Student").iterate();
			while(iter.hasNext()) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
	public void testQuery3() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			List students = session.createQuery("from Student").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			System.out.println("---------------------------------------------");
			
			/**
			 * 不会出现N+1问题
			 * 
			 * 因为list操作已经将Student对象放到了一级缓存中，所以再次使用iterate操作的时候
			 * 它首先发出一条查询id列表的sql，在根据id到缓存中去数据，只有在缓存中找不到相应的
			 * 数据时，才会发出sql到数据库中查询
			 * 
			 */
			Iterator iter = session.createQuery("from Student").iterate();
			while(iter.hasNext()) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}
	
	public void testQuery4() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			List students = session.createQuery("from Student").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			System.out.println("---------------------------------------------");
			
			/**
			 * 再次发出查询sql
			 * 
			 * 在默认情况下list每次都会向数据库发出查询对象的sql，除非配置查询缓存，所以下面的list操作
			 * 虽然在一级缓存中已经有了对象数据，但list默认情况下不会利用缓存，而再次发出sql
			 * 
			 * 默认情况下，list会向缓存中放入数据，但不会利用数据
			 * 
			 */
			students = session.createQuery("from Student").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
}

三、条件查询 
1、可以采用拼字符串的方式传递参数
2、可以采用 ？来传递参数（索引从0开始）
3、可以采用 :参数名 来传递参数
4、如果传递多个参数，可以采用setParamterList方法
5、 在hql中可以使用数据库的函数，如：date_format
 SimpleConditionQueryTest.java  

package com.ncepu.hibernate;

import java.text.SimpleDateFormat;
import java.util.Iterator;
import java.util.List;

import org.hibernate.Query;
import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 条件查询
 * @author Administrator
 *
 */
public class SimpleConditionQueryTest extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//可以拼字符串
			List students = session.createQuery("select s.id, s.name from Student s where s.name like '%1%'").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}
	
	public void testQuery2() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
//			Query query = session.createQuery("select s.id, s.name from Student s where s.name like ?");
//			query.setParameter(0, "%1%");
//			List students = query.list();
			
			//可以使用？方式传递参数
			//参数的索引从0开始
			//传递的参数值，不用单引号引起来
			//注意方法链编程
			List students = session.createQuery("select s.id, s.name from Student s where s.name like ?")
			       					.setParameter(0, "%1%")
			       					.list();
			
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	

	public void testQuery3() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//使用 ：参数名称 的方式传递参数值
			List students = session.createQuery("select s.id, s.name from Student s where s.name like :myname")
			       					.setParameter("myname", "%1%")
			       					.list();
			
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}		
	
	public void testQuery4() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//使用 ：参数名称 的方式传递参数值
			List students = session.createQuery("select s.id, s.name from Student s where s.name like :myname and s.id=:myid")
			       					.setParameter("myname", "%1%")
			       					.setParameter("myid", 12)
			       					.list();
			
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}
	
	public void testQuery5() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//支持in，需要使用setParameterList进行参数传递
			List students = session.createQuery("select s.id, s.name from Student s where s.id in(:myids)")
									.setParameterList("myids", new Object[]{1, 2, 3, 4, 5})
			       					.list();
			
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}			

	public void testQuery6() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//查询2008年2月创建的学生
			List students = session.createQuery("select s.id, s.name from Student s where date_format(s.createTime, '%Y-%m')=?")
									.setParameter(0, "2008-02")
			       					.list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}			

	public void testQuery7() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
			
			//查询2008-01-10到2008-02-15创建的学生
			List students = session.createQuery("select s.id, s.name from Student s where s.createTime between ? and ?")
									.setParameter(0, sdf.parse("2008-01-10 00:00:00"))
									.setParameter(1, sdf.parse("2008-02-15 23:59:59"))
			       					.list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}			
}

四、hibernate也支持直接使用sql进行查询
SqlQueryTest.java

package com.ncepu.hibernate;

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 原生sql测试
 * @author Administrator
 *
 */
public class SqlQueryTest extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			List students = session.createSQLQuery("select * from t_student").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}
	
}

五、外置命名查询
 * 在映射文件中采用<query>标签来定义hql
 * 在程序中采用session.getNamedQuery()方法得到hql查询串
Student.hbm.xml、

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<class name="com.ncepu.hibernate.Student" table="t_student">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<property name="createTime"/>
		<many-to-one name="classes" column="classesid"/>
	</class>
	
	<query name="searchStudents">
		<![CDATA[
			SELECT s FROM Student s where s.id<?
		]]>
	</query>
	</hibernate-mapping>

NameQueryTest.java

package com.ncepu.hibernate;

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 外置命名查询
 * @author Administrator
 *
 */
public class NameQueryTest extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			List students = session.getNamedQuery("searchStudents")
								.setParameter(0, 10)
								.list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
}

六、查询过滤器 
 * 在映射文件中定义过滤器参数
 * 在类的映射中使用这些参数
 * 在程序中启用过滤器
Student.hbm.xml、

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<class name="com.ncepu.hibernate.Student" table="t_student">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<property name="createTime"/>
		<many-to-one name="classes" column="classesid"/>
		<filter name="filtertest" condition="id < :myid"/>
	</class>
	
	<filter-def name="filtertest">
		<filter-param name="myid" type="integer"/>
	</filter-def>	
</hibernate-mapping>

FilterQueryTest.java

package com.ncepu.hibernate;

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 查询过滤器
 * @author Administrator
 *
 */
public class FilterQueryTest extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			session.enableFilter("filtertest")
				    .setParameter("myid", 10);	
			
			List students = session.createQuery("from Student").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
}

七、分页查询 
 * setFirstResult()，从0开始
 * setMaxResults,每页显示多少条数据
PageQueryTest.java

package com.ncepu.hibernate;

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 分页查询
 * @author Administrator
 *
 */
public class PageQueryTest extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			List students = session.createQuery("from Student")
									.setFirstResult(1)
									.setMaxResults(2)
									.list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println(student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}		
}

八、对象导航查询，在hql中采用 . 进行导航
 ObjectNavQueryTest.java

package com.ncepu.hibernate;

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 对象导航查询
 * @author Administrator
 *
 */
public class ObjectNavQueryTest extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			List students = session.createQuery("select s.name from Student s where s.classes.name like '%1%'").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				String name = (String)iter.next();
				System.out.println(name);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
}

九、连接查询
1、内连=join=inner join
2、外连接（左连接/右连接） 

left join(左联接)返回包括左表中的所有记录和右表中联结字段相等的记录
right join(右联接) 返回包括右表中的所有记录和左表中联结字段相等的记录
inner join(等值连接) 只返回两个表中联结字段相等的行

left join
sql语句如下:

select * from A  left join B  on A.aID = B.bID (Hibernate 中配置文件相当于有了A.aID = B.bID)

left join是以A表的记录为基础的,A可以看成左表,B可以看成右表,leftjoin是以左表为准的.换句话说,左表(A)的记录将会全部表示出来,而右表(B)只会显示符合搜索条件的记录(例子中为: A.aID = B.bID).B表记录不足的地方均为NULL.

左连接:  select c.name, s.name from Student s left join c.students s   

right join
sql语句如下:

select * from A right join B  on A.aID = B.bID

以右表(B)为基础的,A表不足的地方用NULL填充.

右连接： select c.name, s.name from Student s left join c.students 

JoinQueryTest.java

package com.ncepu.hibernate;

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 连接查询
 * @author Administrator
 *
 */
public class JoinQueryTest extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			//List students = session.createQuery("select c.name, s.name from Student s join s.classes c").list();
			List students = session.createQuery("select c.name, s.name from Student s inner join s.classes c").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}

	public void testQuery2() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();

			List students = session.createQuery("select c.name, s.name from Classes c left join c.students s").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}
	
	public void testQuery3() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();

			List students = session.createQuery("select c.name, s.name from Classes c right join c.students s").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + "," + obj[1]);
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}
	
}

十、统计查询
StatQueryTest.java

package com.ncepu.hibernate;

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 统计查询
 * @author Administrator
 *
 */
public class StatQueryTest extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
//			List students =session.createQuery("select count(*) from Student").list();
//			Long count = (Long)students.get(0);
//			System.out.println(count);

			Long count = (Long)session.createQuery("select count(*) from Student").uniqueResult();
			System.out.println(count);
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
	public void testQuery2() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			List students =session.createQuery("select c.name, count(s) from Student s join s.classes c " +
					"group by c.name order by c.name").list();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Object[] obj = (Object[])iter.next();
				System.out.println(obj[0] + ", " + obj[1]);
			}
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
}

十一、DML风格的操作（尽量少用，因为和缓存不同步）

DML = Data Manipulation Language，数据操纵语言，命令使用户能够查询数据库以及操作已有数据库中的数据的计算机语言。具体是指是UPDATE更新、INSERT插入、DELETE删除。

DMLQueryTest.java

package com.ncepu.hibernate;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * DML风格的操作
 * @author Administrator
 *
 */
public class DMLQueryTest extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			session.createQuery("update Student s set s.name=? where s.id < ?")
					.setParameter(0, "李四")
					.setParameter(1, 5)
					.executeUpdate();
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}		
}

应 当尽量少用，因为和缓存不同步，也就是说，假如在执行上面的语句之前，已经把student封装成一个list曾经拿了出来，再执行上面的语句对student中的表进行数据更新，然后再list　Student表，则此时的list是从缓存中取的数据，而不是从表中找到的数据，也就是 说，list拿到的是update前的数据，所以造成了这种不同步，所以这种风格尽量少用。
从这个方面也可以看得出hibernate并不适用于聚集性，统计，大量批量的更新，删除等操作