【Hibernate】Hibernate的多表查询

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在《【hibernate】Hibernate的聚类查询、分组查询、排序与时间之差》（点击打开链接）一文中已经讲述过如何利用HQL语句取代SQL语句，进行聚类查询、分组查询、排序与时间之差的查询，同时指出hql能代替sql语句做任何事情。我原本以为hql语句的多表查询，要先对里面的实体做Java与xml的修改，其实并不需要，同样是一句HQL语句就能搞定的事情。SQL的多表查询已经在《【MySQL】利用内连接与嵌套查询实现多表查询，主键、外键的基本概念》（点击打开链接）讲过。

比如如下的SQL语句：

[sql] view plain copy print?

select t1.Title,t1.Content
from blog as t1 ,usertable as t2
where t1.userid=t2.id and t2.username='a'

转换成HQL语句则如下：

[java] view plain copy print?

String hql="select t1.title,t1.content from Blog as t1,Usertable as t2 where t1.userId=t2.id and t2.username='a'"
List<Object> resultList = session.createQuery(hql).list();
for (int i = 0; i < resultList.size(); i++) {
Object[] obj = (Object[])resultList.get(i);
System.out.println(obj[0]+","+obj[1]);
}

核心思想，是把sql语句中的表，写成Hibernate的实体，sql语句中的字段、列，写成Hibernate的实体的成员变量，同时必须把表使用as进行t1,t2等标记，不能使用Blog.XX字段来简化，如上，就是Blog,Usertable两张表转化为t1,t2标记，把sql的字段、列，表对应转化为Hibernate的实体成员变量，实体进行查询。

Hibernate查询出来的结果是一个存放Object数组的List，也就是说List的每项都是一个Object数组，Object数组的第n项对应查询结果的第n项。

可以再进行下一步的处理。

下面用一个例子，来说明HQL语句的多表查询。

如图，Blog记录了用户发表的博客，usertable记录了用户的基本信息。Blog表中的userid与usertable的主键id形成参照完整性。

这两张表在Hibernate的Java工程种分别对应如下实体：

Blog.java

[java] view plain copy print?

import javax.persistence.*;
@Entity
@Table(name = "blog")
public class Blog {
private int id;
private String title;
private String content;
private int userId;
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
@Column(name = "Title")
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
@Column(name = "Content")
public String getContent() {
return content;
}
public void setContent(String content) {
this.content = content;
}
@Column(name = "userid")
public int getUserId() {
return userId;
}
public void setUserId(int userId) {
this.userId = userId;
}
@Override
public String toString() {
return id + "," + title + "," + content + "," + userId;
}
}

Usertable.java

[java] view plain copy print?

import javax.persistence.*;
@Entity
@Table(name = "usertable")
public class Usertable {
private int id;
private String username;
private String password;
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
@Column(name = "username")
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
@Column(name = "password")
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
@Override
public String toString() {
return id + "," + username + "," + password;
}
}

同时，hibernate.cfg.xml做如下的配置：

[html] view plain copy print?

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.password">admin</property>
<property name="hibernate.connection.url">jdbc:mysql://localhost:3306/test</property>
<property name="hibernate.connection.username">pc</property>
<property name="hibernate.dialect">org.hibernate.dialect.MySQLInnoDBDialect</property>
<property name="hibernate.format_sql">true</property>
<property name="hibernate.hbm2ddl.auto">update</property>
<mapping class="Usertable" />
<mapping class="Blog" />
</session-factory>
</hibernate-configuration>

可以发现，这没有进行任何的参照完整性的指定。下面，要查询usertable中username为a的用户，发表的Blog。

我们知道先要查询username为a的用户的id，之后利用这个查出来的id到Blog表中查询。

用sql语句完成这个多表查询，则这样写：

[sql] view plain copy print?

select t1.Title,t1.Content
from blog as t1 ,usertable as t2
where t1.userid=t2.id and t2.username='a'

其查询结果如下：

这使用Hibernate则这样写，在HibernateMultiTableTest.java中的代码如下：

[java] view plain copy print?

import java.util.List;
import org.hibernate.*;
import org.hibernate.cfg.*;
class dbDAO {
private Session session;
// 构造函数，初始化Session，相当于连接数据库
public dbDAO() {
// new Configuration().configure()是吧hibernate.cfg.xml中的所有配置读取进来
// .buildSessionFactory().openSession()是创建Session工厂并实例化session
this.session = new Configuration().configure().buildSessionFactory()
.openSession();
}
// 执行查询
public Query query(String hql) {
return session.createQuery(hql);
}
// 执行插入、修改
public void save(Object object) {
Transaction transaction = session.beginTransaction();
session.save(object);
transaction.commit();
}
// 执行删除
public void delete(Object object) {
Transaction transaction = session.beginTransaction();
session.delete(object);
transaction.commit();
}
// 析构函数，中断Session，相当于中断数据库的连接
protected void finalize() throws Exception {
if (session.isConnected() || session != null) {
session.close();
}
}
}
@SuppressWarnings("unchecked")
public class HibernateMultiTableTest {
public static void main(String args[]) {
dbDAO db = new dbDAO();
List<Object> resultList = db
.query("select t1.title,t1.content from Blog as t1,Usertable as t2 where t1.userId=t2.id and t2.username='a'")
.list();//HQL的多表查询
System.out.println("usertable中username为a的用户，发表的内容如下：");
System.out.println();
for (int i = 0; i < resultList.size(); i++) {
Object[] obj = (Object[]) resultList.get(i);
System.out.println("标题：" + obj[0]);
System.out.println("内容：" + obj[1]);
System.out.println();
}
}
}

其运行结果如下：

转载自：yongh701

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