mybatis 详解(七)------一对一、一对多、多对多
前面几篇博客我们用mybatis能对单表进行增删改查操作了,也能用动态SQL书写比较复杂的sql语句。但是在实际开发中,我们做项目不可能只是单表操作,往往会涉及到多张表之间的关联操作。那么我们如何用 mybatis 处理多表之间的关联操作呢?请看本篇博客详解。
本篇详细代码:http://pan.baidu.com/s/1eSzmst8 密码:3n3o
1、一对一
我们以用户表 user 和订单表 orders 为例。设定一个订单只能由一个 用户创建,那么由订单到用户就是一对一的关系。
①、创建用户表 user 和订单表 orders
用户表 user
订单表 orders
②、创建项目工程,导入相应的 jar 包
③、创建实体类
User.java
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package com.ys.po;
public class User {
//用户ID
private int id;
//用户姓名
private String username;
//用户性别
private String sex;
public int getId() {
return id;
}
public void setId( int id) {
this .id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this .username = username;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this .sex = sex;
}
@Override
public String toString() {
return "User [id=" + id + ", username=" + username + ", sex=" + sex
+ "]" ;
}
} |
Orders.java
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package com.ys.po;
public class Orders {
//订单ID
private int id;
//用户ID
private int userId;
//订单数量
private String number;
//和用户表构成一对一的关系,即一个订单只能由一个用户创建
private User user;
public int getId() {
return id;
}
public void setId( int id) {
this .id = id;
}
public int getUserId() {
return userId;
}
public void setUserId( int userId) {
this .userId = userId;
}
public String getNumber() {
return number;
}
public void setNumber(String number) {
this .number = number;
}
public User getUser() {
return user;
}
public void setUser(User user) {
this .user = user;
}
@Override
public String toString() {
return "Orders [id=" + id + ", userId=" + userId + ", number=" + number
+ ", user=" + user + "]" ;
}
} |
④、创建 OrderMapper 接口和 OrderMapper.xml 文件
由于我们采用 Mapper 代理加载 xxxMapper.xml 文件,这里我们重复一下 Mapper 代理所需的条件,接口和xml文件必须满足以下几个条件:
1、接口必须要和 xml 文件同名且在同一个包下,也就是说 xml 文件中的namespace是接口的全类名
2、接口中的方法名和xml 文件中定义的 id 一致
3、接口输入参数类型要和xml 中定义的 parameterType 一致
4、接口返回数据类型要和xml 中定义的 resultType 一致
详细介绍参考上一篇博客:http://www.cnblogs.com/ysocean/p/7301548.html
OrderMapper 接口
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package one.to.one.mapper;
import com.ys.po.Orders;
import com.ys.po.User;
public interface OrdersMapper {
/**
* 方式一:嵌套结果
* select * from orders o,user u where o.user_id=u.id and o.id=#{id}
* @param orderId
* @return
*/
//根据订单ID查询订单和用户信息
public Orders selectOrderAndUserByOrderID( int orderId);
/**
* 方式二:嵌套查询
* select * from order WHERE id=1;//得到user_id
* select * from user WHERE id=1 //1 是上一个查询得到的user_id的值
* @param userID
* @return
*/
//根据订单ID得到订单信息(包含user_id)
public Orders getOrderByOrderId( int orderId);
//根据用户ID查询用户信息
public User getUserByUserId( int userID);
} |
OrderMapper .xml文件
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<?xml version= "1.0" encoding= "UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace= "one.to.one.mapper.OrdersMapper" >
<!--
嵌套结果:使用嵌套结果映射来处理重复的联合结果的子集
封装联表查询的数据(去除重复的数据)
select * from orders o,user u where o.user_id=u.id and o.id=#{id}
-->
<select id= "selectOrderAndUserByOrderID" resultMap= "getOrderAndUser" >
select * from orders o,user u where o.user_id=u.id and o.id=#{id}
</select>
<resultMap type= "com.ys.po.Orders" id= "getOrderAndUser" >
<!--
id:指定查询列表唯一标识,如果有多个唯一标识,则配置多个id
column:数据库对应的列
property:实体类对应的属性名
-->
<id column= "id" property= "id" />
<result column= "user_id" property= "userId" />
<result column= "number" property= "number" />
<!--association:用于映射关联查询单个对象的信息
property:实体类对应的属性名
javaType:实体类对应的全类名
-->
<association property= "user" javaType= "com.ys.po.User" >
<!--
id:指定查询列表唯一标识,如果有多个唯一标识,则配置多个id
column:数据库对应的列
property:实体类对应的属性名
-->
<id column= "id" property= "id" />
<result column= "username" property= "username" />
<result column= "sex" property= "sex" />
</association>
</resultMap>
<!--
方式二:嵌套查询:通过执行另外一个SQL映射语句来返回预期的复杂类型
select user_id from order WHERE id= 1 ; //得到user_id
select * from user WHERE id= 1 //1 是上一个查询得到的user_id的值
property:别名(属性名) column:列名 -->
<select id= "getOrderByOrderId" resultMap= "getOrderMap" >
select * from order where id=#{id}
</select>
<resultMap type= "com.ys.po.Orders" id= "getOrderMap" >
<id column= "id" property= "id" />
<result column= "number" property= "number" />
<association property= "userId" column= "id" select= "getUserByUserId" >
</association>
</resultMap>
<select id= "getUserByUserId" resultType= "com.ys.po.User" >
select * from user where id=#{id}
</select>
</mapper> |
⑤、向 mybatis-configuration.xml 配置文件中注册 OrderMapper.xml 文件
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<?xml version= "1.0" encoding= "UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd" >
<configuration> <!-- 加载数据库属性文件 --> <properties resource= "db.properties" ></properties>
<!-- 定义别名 --> <typeAliases> <!-- mybatis自动扫描包中的po类,自动定义别名,别名是类名(首字母大写或小写都可以,一般用小写) -->
< package name= "com.ys.po" />
</typeAliases> <environments default = "development" >
<environment id= "development" >
<transactionManager type= "JDBC" />
<!--dataSource 元素使用标准的 JDBC 数据源接口来配置 JDBC 连接对象源 -->
<dataSource type= "POOLED" >
<property name= "driver" value= "${jdbc.driver}" />
<property name= "url" value= "${jdbc.url}" />
<property name= "username" value= "${jdbc.username}" />
<property name= "password" value= "${jdbc.password}" />
</dataSource>
</environment>
</environments>
<mappers>
<!-- 通过OrdersMapper接口注册OrdersMapper.xml文件,
必须保证:接口和xml在同一个包下,而且名字一样
OrdersMapper接口的方法名和OrdersMapper.xml文件的id一样
OrdersMapper接口的输出输出参数和OrdersMapper.xml文件resultType,parameterType类型一样
-->
<mapper class = "one.to.one.mapper.OrdersMapper" />
</mappers>
</configuration> |
⑥、测试
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package one.to.one.mapper;
import java.io.InputStream;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;
import com.ys.po.Orders;
public class OneToOneTest {
//定义 SqlSession
SqlSession session = null ;
@Before
public void init(){
//定义mybatis全局配置文件
String resource = "mybatis-configuration.xml" ;
//加载 mybatis 全局配置文件
InputStream inputStream = OneToOneTest. class .getClassLoader()
.getResourceAsStream(resource);
//构建sqlSession的工厂
SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//根据 sqlSessionFactory 产生 session
session = sessionFactory.openSession();
}
/**
* 方式一:嵌套结果
* select * from orders o,user u where o.user_id=u.id and o.id=#{id}
*/
@Test
public void testSelectOrderAndUserByOrderId(){
String statement = "one.to.one.mapper.OrdersMapper.selectOrderAndUserByOrderID" ;
//创建OrdersMapper对象,mybatis自动生成mapepr代理对象
OrdersMapper orderMapper = session.getMapper(OrdersMapper. class );
Orders order = orderMapper.selectOrderAndUserByOrderID( 1 );
System.out.println(order);
session.close();
}
/**
* 方式二:嵌套查询
* select * from order WHERE id=1;//得到user_id
* select * from user WHERE id=1 //1 是上一个查询得到的user_id的值
*/
@Test
public void testgetOrderByOrderId(){
String statement = "one.to.one.mapper.OrdersMapper.getOrderByOrderId" ;
//创建OrdersMapper对象,mybatis自动生成mapepr代理对象
OrdersMapper orderMapper = session.getMapper(OrdersMapper. class );
Orders order = orderMapper.selectOrderAndUserByOrderID( 1 );
System.out.println(order);
session.close();
}
} |
2、一对多
还是以用户表 user 和 订单表 orders 为例,一个用户能创建多个订单。故用户和订单构成一对多的关联。
我们在 user.java 中添加一个属性 public List<Orders> orders;
①、创建实体类
user.java如下,orders.java保持不变
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package com.ys.po;
import java.util.List;
public class User {
//用户ID
private int id;
//用户姓名
private String username;
//用户性别
private String sex;
//一个用户能创建多个订单,用户和订单构成一对多的关系
public List<Orders> orders;
public List<Orders> getOrders() {
return orders;
}
public void setOrders(List<Orders> orders) {
this .orders = orders;
}
public int getId() {
return id;
}
public void setId( int id) {
this .id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this .username = username;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this .sex = sex;
}
@Override
public String toString() {
return "User [id=" + id + ", username=" + username + ", sex=" + sex
+ "]" ;
}
} |
②、创建 UserMapper 接口和 UserMapper.xml 文件
UserMapper接口
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package one.to.many.mapper;
import com.ys.po.User;
public interface UserMapper {
//根据用户id查询用户信息,以及用户下面的所有订单信息
public User selectUserAndOrdersByUserId( int UserId);
} |
UserMapper.xml
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<?xml version= "1.0" encoding= "UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace= "one.to.many.mapper.UserMapper" >
<!--
方式一:嵌套结果:使用嵌套结果映射来处理重复的联合结果的子集
封装联表查询的数据(去除重复的数据)
select * from user u,orders o where u.id=o.user_id and u.id=#{id}
-->
<select id= "selectUserAndOrdersByUserId" resultMap= "getUserAndOrders" >
select u.*,o.id oid,o.number number from user u,orders o where u.id=o.user_id and u.id=#{id}
</select>
<resultMap type= "com.ys.po.User" id= "getUserAndOrders" >
<!--id:指定查询列表唯一标识,如果有多个唯一标识,则配置多个id
column:数据库对应的列
property:实体类对应的属性名 -->
<id column= "id" property= "id" />
<result column= "username" property= "username" />
<result column= "sex" property= "sex" />
<!--
property:实体类中定义的属性名
ofType:指定映射到集合中的全类名
-->
<collection property= "orders" ofType= "com.ys.po.Orders" >
<id column= "oid" property= "id" />
<result column= "number" property= "number" />
</collection>
</resultMap>
</mapper> |
③、向 mybatis-configuration.xml 配置文件中注册 UserMapper.xml 文件
④、测试
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@Test public void testSelectOrderAndUserByOrderId(){
String statement = "one.to.many.mapper.UserMapper.selectUserAndOrdersByUserId" ;
//创建OrdersMapper对象,mybatis自动生成mapepr代理对象
UserMapper userMapper = session.getMapper(UserMapper. class );
User user = userMapper.selectUserAndOrdersByUserId( 1 );
System.out.println(user.getOrders().size());
session.close();
} |
3、多对多
这里我们以用户 user 表和 角色role 表为例,假定一个用户能被分配成多重角色,而一种角色也能分给多个用户,故用户和角色构成多对多的关系。
需求:给定角色id,查询这个角色所属的所有用户信息
①、在数据库中建立相应的表
user 表和上面的保持不变
role 表
两者之间的关联表user_role
②、建立对应的实体类
User.java
Role.java
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package com.ys.po;
import java.util.List;
public class Role {
private int id;
private String name;
private List<User> users;
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 List<User> getUsers() {
return users;
}
public void setUsers(List<User> users) {
this .users = users;
}
} |
User_Role.java
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package com.ys.po;
public class User_Role {
private User user;
private Role role;
public User getUser() {
return user;
}
public void setUser(User user) {
this .user = user;
}
public Role getRole() {
return role;
}
public void setRole(Role role) {
this .role = role;
}
} |
③、创建 UserMapper 接口和 UserMapper.xml 文件
UserMapper 接口
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package many.to.many.mapper;
import java.util.List;
import com.ys.po.User;
public interface UserMapper {
//给定一个角色id,要得到具有这个角色的所有用户信息
public List<User> getUserByRoleId( int roleId);
} |
UserMapper.xml
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<?xml version= "1.0" encoding= "UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace= "many.to.many.mapper.UserMapper" >
<select id= "getUserByRoleId" resultMap= "getUserMap" >
select * from user_role ur,user u where ur.user_id=u.id and ur.role_id=#{id}
</select>
<resultMap type= "com.ys.po.User" id= "getUserMap" >
<id column= "id" property= "id" />
<result column= "username" property= "username" />
<result column= "sex" property= "sex" />
</resultMap>
</mapper> |
④、向 mybatis-configuration.xml 配置文件中注册 UserMapper.xml 文件
⑤、测试
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@Test public void testGetUserByRoleId(){
String statement = "many.to.many.mapper.UserMapper.getUserByRoleId" ;
//创建OrdersMapper对象,mybatis自动生成mapepr代理对象
UserMapper userMapper = session.getMapper(UserMapper. class );
List<User> users = userMapper.getUserByRoleId( 1 );
session.close();
} |
多对多主要是关联关系要找好,然后根据关联去查询。