Hibernate HQL示例八:查询对象导航及Implicit Join
通过Student对象导航到class对象的id如下：
POJO
package com.bjsxt.hibernate;import java.util.Date;public class Student {private int id;private String name; private Date createTime;private Classes classes;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Classes getClasses() {return classes;}public void setClasses(Classes classes) {this.classes = classes;}public Date getCreateTime() {return createTime;}public void setCreateTime(Date createTime) {this.createTime = createTime;}}
package com.bjsxt.hibernate;import java.util.Set;public class Classes {private int id;private String name;private Set students;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public Set getStudents() {return students;}public void setStudents(Set students) {this.students = students;}}
<?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.bjsxt.hibernate.Classes" table="t_classes"><id name="id"><generator class="native"/></id><property name="name"/><!-- <set name="students" cascade="all" order-by="id"> --> <set name="students" inverse="true"><key column="classid"/><one-to-many class="com.bjsxt.hibernate.Student"/></set></class></hibernate-mapping>
<?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.bjsxt.hibernate.Student" table="t_student"><id name="id"><generator class="native"/></id><property name="name"/><property name="createTime"/><many-to-one name="classes" column="classid"/><filter name="idfilter" condition="id &lt; :myid"/></class><query name="searchStudents"><!]></query><filter-def name="idfilter"><filter-param name="myid" type="integer"/></filter-def></hibernate-mapping>
下面测试
package com.bjsxt.hibernate;import java.util.Iterator;import java.util.List;import org.hibernate.Session;import junit.framework.TestCase;/** * 对象导航查询测试 * @author Administrator * */public class ObjectNavQueryTest extends TestCase {public void testQuery1() {Session session = null;try {session = HibernateUtils.getSession();List students = session.createQuery("select s.name from Student s where s.classes.id < 2").list();for (Iterator iter = students.iterator();iter.hasNext();) {String name = (String)iter.next();System.out.println(name);}}catch(Exception e) {e.printStackTrace();}finally {HibernateUtils.closeSession(session);}}}
上面select s.name from Student s where s.classes.id < 2实际上使用了隐式的inner join
对应的sql语句有两种形式
mysql> select s.name from t_student s ,t_classes c where s.classid = c.id and c.id<2;mysql> select t.name from t_student t inner join t_classes c on t.classid=c.id where c.id<2;
对象导航其实就是一种隐式join的应用。
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