MyBatis映射（一对多查询）
目录
1.一对多查询
2.1 数据库准备
代码：
USE mybatis;
# 创建一个名称为tb_user的表
CREATE TABLE tb_user (
id int(32) PRIMARY KEY AUTO_INCREMENT,
username varchar(32),
address varchar(256)
);
# 插入3条数据
INSERT INTO tb_user VALUES ('1', '小明', '北京');
INSERT INTO tb_user VALUES ('2', '李华', '上海');
INSERT INTO tb_user VALUES ('3', '李刚', '上海');
# 创建一个名称为tb_orders的表
CREATE TABLE tb_orders (
id int(32) PRIMARY KEY AUTO_INCREMENT,
number varchar(32) NOT NULL,
user_id int(32) NOT NULL,
FOREIGN KEY(user_id) REFERENCES tb_user(id)
);
# 插入3条数据
INSERT INTO tb_orders VALUES ('1', '1000011', '1');
INSERT INTO tb_orders VALUES ('2', '1000012', '1');
INSERT INTO tb_orders VALUES ('3', '1000013', '2');
2.2 实体类
2.2.1 Orders.java
代码：
package com.biem.pojo;
import lombok.*;
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@ToString
public class Orders {
private Integer id;
private String number;
private Integer userId;
}
2.2.2 Users.java
代码：
package com.biem.pojo;
import lombok.*;
import java.util.List;
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@ToString
public class Users {
private Integer id;
private String username;
private String address;
private List<Orders> ordersList;
}
2.3 用户配置接口
2.3.1 OrdersMapper.java
代码：
package com.biem.mapper;
public interface OrdersMapper {
}
2.3.2 UsersMapper.java
代码：
package com.biem.mapper;
public interface UsersMapper {
}
2.4 用户配置文件
2.4.1 OrdersMapper.xml
代码：
<?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="com.biem.mapper.OrdersMapper">
</mapper>
2.4.2 UsersMapper.xml
代码：
<?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="com.biem.mapper.UsersMapper">
</mapper>
2.5 解决办法
2.5.1 嵌套查询方式
2.5.1.1 OrdersMapper.java
代码：
package com.biem.mapper;
public interface OrdersMapper {
public List<Orders> selectOrders(Integer uid);
}
2.5.1.2 UsersMapper.java
代码：
package com.biem.mapper;
import com.biem.pojo.User;
public interface UserMapper {
public User findUserById(int id);
}
2.5.1.3 OrdersMapper.xml（见上图）
代码：
<?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="com.biem.mapper.OrdersMapper">
<!-- public List<Orders> selectOrders(Integer user_id); -->
<select id="selectOrders" parameterType="Integer" resultType="Orders">
select * from tb_orders where user_id=#{userId}
</select>
</mapper>
2.5.1.4 UsersMapper.xml（见上图）
代码：
<?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="com.biem.mapper.UsersMapper">
<resultMap id="usersWithOrders1" type="Users">
<id property="id" column="id"></id>
<result property="username" column="username"/>
<result property="address" column="address"/>
<!--由于查询老师中有一个属性为所有学生对象集合，所一使用collection标签进行嵌套，
方法的返回值是List，所以javaType使用ArrayList来接收，而list中泛型约束类型放的
是orders对象所以使用ofType为orderst来进行映射，使用select进行子查询，并用column
将查询出的id字段传给子查询需要的id-->
<!--此处javaType可以省略-->
<collection property="ordersList" javaType="ArrayList" ofType="orders" select="com.biem.mapper.OrdersMapper.selectOrders" column="id"/>
</resultMap>
<!-- public Users findUserById(Integer id); -->
<select id="findUserById" parameterType="Integer" resultMap="usersWithOrders1">
select * from tb_user where id = #{id}
</select>
</mapper>
2.5.1.5 UsersTest.java
代码：
import org.junit.Test;
public class UsersTest {
@Test
public void testFindUserById(){
SqlSession session = MyBatisUtil.openSession();
UsersMapper mapper = session.getMapper(UsersMapper.class);
Users user = mapper.findUserById(1);
System.out.println("user = " + user);
session.close();
}
}
2.5.1.6 测试结果
2.5.2 嵌套结果方式
2.5.2.1 UsersMapper.java添加
代码：
public Users findUserByIdWithNestedResults(Integer id);
2.5.2.2 UsersMapper.xml添加
代码：
<!-- public Users findUserByIdWithNestedResults(Integer id); -->
<!-- 一对多：查看某一个用户及关联的订单信息，
注意：当关联查询出来的列名相同时，则需要使用别名区分 -->
<select id="findUserByIdWithNestedResults" parameterType="Integer" resultMap="UsersWithOrdersResult" >
select u.*, o.id as orders_id , o.number, o.user_id
from tb_user u, tb_orders o
where u.id = o.user_id and u.id=#{id}
</select>
<resultMap id="UsersWithOrdersResult" type="Users">
<id property="id" column="id"></id>
<result property="username" column="username"/>
<result property="address" column="address"/>
<collection property="ordersList" ofType="Orders">
<id property="id" column="orders_id"></id>
<result property="number" column="number"/>
<result property="userId" column="user_id"/>
</collection>
</resultMap>
2.5.2.3 UsersTest.java添加
代码：
@Test
public void testFindUserByIdWithNestedResults(){
SqlSession session = MyBatisUtil.openSession();
UsersMapper mapper = session.getMapper(UsersMapper.class);
Users user = mapper.findUserByIdWithNestedResults(1);
System.out.println("user = " + user);
session.close();
}
2.5.2.4 测试结果
