// 假设有一个User实体类和一个Order实体类，以及相应的数据库表
public class User {
private Integer id;
private String username;
// 省略其他属性、构造函数、getter和setter
}
public class Order {
private Integer id;
private Integer userId;
private String orderNumber;
// 省略其他属性、构造函数、getter和setter
}
// 映射文件UserMapper.xml
<?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.example.mapper.UserMapper">
<!-- 定义结果映射 -->
<resultMap id="UserOrderResultMap" type="com.example.entity.User">
<id property="id" column="user_id"/>
<result property="username" column="username"/>
<!-- 定义关联的订单信息 -->
<collection property="orders" ofType="com.example.entity.Order">
<id property="id" column="order_id"/>
<result property="userId" column="user_id"/>
<result property="orderNumber" column="order_number"/>
</collection>
</resultMap>
<!-- 使用结果映射的查询多表 -->
<select id="selectUserWithOrders" resultMap="UserOrderResultMap">
SELECT u.id AS user_id, u.username, o.id AS order_id, o.user_id, o.order_number
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
WHERE u.id = #{id}
</select>
</mapper>
// UserMapper接口
public interface UserMapper {
User selectUserWithOrders(Integer id);
}
// 使用MyBatis注解开发，无需XML映射文件
public interface UserMapper {
@Select("SELECT u.id, u.username, o.id AS order_id, o.user_id, o.order_number " +
"FROM users u " +
"LEFT JOIN orders o ON u.id = o.user_id " +
"WHERE u.id = #{id}")
@Results({
@Result(property = "id", column = "id"),
@Result(property = "username", column = "username"),
@Result(property = "orders", column = "order_id", javaType = List.class,
many = @Many(select = "selectOrdersByUserId"))
})
User selectUserWithOrders(Integer id);
@Select("SELECT id, user_id, order_number FROM orders WHERE user_id = #{userId}")
List<Order> selectOrdersByUserId(Integer userId);
}
在这个例子中，我们展示了如何使用MyBatis进行多表查询，并且如何使用结果映射<resultMap>以及注解@Results和@Many来简化结果的映射处理。这样的