一对多关联映射
一对多关联映射有两种方式,都用到了collection元素
以购物网站中用户和订单之间的一对多关系为例
collection集合的嵌套结果映射
创建两个实体类和映射接口
package org.example.demo;
import lombok.Data;
import java.util.List;
@Data
public class User {
private Integer userId;
private String userName;
private String password;
private Cart cart;
private List<Order> orderList;
}
package org.example.demo;
import lombok.Data;
@Data
public class Order {
private Integer orderId;
private double price;
}
实现根据用户id查询出所有用户信息,包括该用户的所有订单信息
package org.example.mapper;
import org.example.demo.User;
import java.util.List;
public interface UserMapper {
User findUserAndOrderListByUserId(Integer userId);
}
方式一:与association类似,集合的嵌套结果映射就是指通过一次SQL查询得到所有的结果
<resultMap type="com.mybatis.entity.User"
id="userMap">
<id property="id" column="id"/>
<result property="userName" column="user_name"/>
<result property="password" column="password"/>
</resultMap>
<resultMap type="com.mybatis.entity.User"
id="userAndOrderListMap" extends="userMap">
<collection property="orderList"
ofType="com.mybatis.entity.Order">
<id property="id" column="order_id"/>
<result property="price" column="price"/>
</collection>
</resultMap>
<select id="findUserAndOrderListById"
resultMap="userAndOrderListMap">
select u.id, u.user_name, u.password,
o.order_id, o.price
from user u
left join orders o on u.id = o.user_id
where u.id = #{id}
</select>
resultMap元素中的extends属性可以实现结果映射的继承
collection的ofType属性指定集合中元素的类型,必选项
<resultMap id="userAndOrderMap" type="org.example.demo.User">
<id property="userId" column="user_id"/>
<result property="userName" column="user_name"/>
<result property="password" column="password"/>
<collection property="orderList" ofType="org.example.demo.Order">
<id property="orderId" column="order_id"/>
<result property="price" column="price"/>
</collection>
</resultMap>
<select id="findUserAndOrderListByUserId" resultMap="userAndOrderMap">
select
*
from t_user u
left join t_order o on u.user_id = o.user_id
where u.user_id = #{userId};
</select>
collection集合的嵌套查询
集合的嵌套查询同样会执行额外的SQL查询
<resultMap type="com.mybatis.entity.User"
id="userAndOrderListMap" extends="userMap">
<collection property="orderList" column="{uid=id}"
ofType="com.mybatis.entity.Order" select="com.mybatis.mapper.OrderMapper.findOrdersByUserId">
</collection>
</resultMap>
<select id="findUserAndOrderListById"
resultMap="userAndOrderListMap">
select * from user where id = #{id}
</select>
OrderMapper.xml
<resultMap type="com.mybatis.entity.Order"
id="orderMap">
<id property="id" column="order_id"/>
<result property="price" column="price"/>
</resultMap>
<select id="findOrdersByUserId"
resultMap="orderMap">
select * from orders where user_id = #{uid}
</select>
对比两种方式
第一种方式属于“关联的嵌套结果映射“,即通过一次SQL查询根据表或指定的属性映射到不同的对象中
第二种方式属于“关联的嵌套查询”,利用简单的SQL语句,通过多次查询得到想要的结果,也可以实现延迟加载效果
标签:映射,private,关联,嵌套,user,Mybatis,查询,id From: https://blog.csdn.net/LIUCHANGSHUO/article/details/137266856