目录
[2.1. 订单实体类](#2.1. 订单实体类)
[2.2. 用户实体类](#2.2. 用户实体类)
[2.3. OrderMapper.xml](#2.3. OrderMapper.xml)
[2.4. sqlMapConfig.xml](#2.4. sqlMapConfig.xml)
[2.5. properties](#2.5. properties)
[2.6. OrderMapper](#2.6. OrderMapper)
[2.7. UserService](#2.7. UserService)
一、引言
在我们日常开发中，经常会碰到多表连接查询的业务场景，比如查询某一笔订单（包含所属用户信息），这种我们称之为一对一查询。在MyBatis中，一对一查询通常是通过配置映射文件中的<resultMap>来实现的。
一对一查询模型
用户表和订单表的关系为：
一个用户有多个订单，一个订单只从属于一个用户，订单表uid关联用户表id。
一对一查询的需求：查询一个订单，与此同时查询出该订单所属的用户。
二、完整代码实现（1）
2.1. 订单实体类
java
package com.example.domain;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import java.util.Date;
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Order {
private int id;
private Date orderTime;
private double total;
// 代表当前订单从属于哪一个用户
private User user;
}2.2. 用户实体类
java
package com.example.domain;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;
import java.util.Date;
@Data
@AllArgsConstructor
@NoArgsConstructor
@ToString
public class User {
private int id;
private String username;
private String password;
private Date birthday;
}2.3. OrderMapper.xml
XML
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.mapper.OrderMapper">
<resultMap id="orderMap" type="com.example.domain.Order">
<!-- 手动指定字段与实体属性的映射关系
column: 数据表的字段名称
property: 实体的属性名称
-->
<id column="oid" property="id"></id>
<result column="ordertime" property="orderTime"></result>
<result column="total" property="total"></result>
<result column="uid" property="user.id"></result>
<result column="username" property="user.username"></result>
<result column="password" property="user.password"></result>
<result column="birthday" property="user.birthday"></result>
</resultMap>
<!-- 查询操作 -->
<select id="findAll" resultMap="orderMap">
select *, o.id oid from `order` o inner join USER u on o.uid=u.id
</select>
</mapper>2.4. sqlMapConfig.xml
XML
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<!-- 加载外部properties -->
<properties resource="jdbc.properties"></properties>
<settings>
<!-- 其他配置 -->
<setting name="logImpl" value="STDOUT_LOGGING"/>
</settings>
<typeAliases>
<typeAlias type="com.example.domain.User" alias="user"></typeAlias>
</typeAliases>
<plugins>
<plugin interceptor="com.github.pagehelper.PageHelper">
<!-- 指定方言 MySQl -->
<property name="dialect" value="mysql"/>
</plugin>
</plugins>
<!-- 数据源环境 -->
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<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>
<!-- 加载映射文件 -->
<mappers>
<mapper resource="com/example/mapper/UserMapper.xml"/>
<mapper resource="com/example/mapper/OrderMapper.xml"/>
</mappers>
</configuration>2.5. properties
XML
jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/test
jdbc.username=root
jdbc.password=Wangzhexiao2.6. OrderMapper
java
package com.example.mapper;
import com.example.domain.Order;
import java.io.IOException;
import java.util.List;
public interface OrderMapper {
List<Order> findAll() throws IOException;
}2.7. UserService
java
package com.example.service;
import com.example.domain.Order;
import com.example.mapper.OrderMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
public class UserService {
public static void main(String[] args) throws IOException {
// 获得核心配置文件
InputStream inputStream = Resources.getResourceAsStream("sqlMapConfig.xml");
// 获得Session工厂对象
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
// 获得Session会话对象
SqlSession sqlSession = sqlSessionFactory.openSession();
// 获取Dao代理对象，Mybatis内部已经帮我们完成了实现（与我们传统方式代码中的实现类相似）
OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
List<Order> list = mapper.findAll();
for (Order order : list) {
System.out.println(order);
}
}
}三、代码实现（2）
我们在上个代码的基础上做适当调整：
XML
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.mapper.OrderMapper">
<resultMap id="orderMap" type="com.example.domain.Order">
<!-- 手动指定字段与实体属性的映射关系
column: 数据表的字段名称
property: 实体的属性名称
-->
<id column="oid" property="id"></id>
<result column="ordertime" property="orderTime"></result>
<result column="total" property="total"></result>
<!-- <result column="uid" property="user.id"></result>-->
<!-- <result column="username" property="user.username"></result>-->
<!-- <result column="password" property="user.password"></result>-->
<!-- <result column="birthday" property="user.birthday"></result>-->
<association property="user" javaType="user">
<result column="uid" property="id"></result>
<result column="username" property="username"></result>
<result column="password" property="password"></result>
<result column="birthday" property="birthday"></result>
</association>
</resultMap>
<!-- 查询操作 -->
<select id="findAll" resultMap="orderMap">
select *, o.id oid from `order` o inner join USER u on o.uid=u.id
</select>
</mapper>