【Mybatis】一对一与一对多的关联查询

一、一对一关联查询

背景：

表1：user包含id，username。

表2：orders包含id，user_id

需求：查询每个用户下的订单ID。( 现实中是一对多的关系，即一个用户对应着多个订单，主要是学习mybatis，这里只是借用举例)

sql：

select 
u.id,
u.username,
o.id
from `user` as u
LEFT JOIN orders as o on u.id = o.user_id

方法一：通过继承，类的组合，对应于需求要查询的信息。如继承user类中再加入orders的对象

package com.test.domain;

public class UserOrders extends User {

	private orders order;

	public orders getOrder() {
		return order;
	}

	public void setOrder(orders order) {
		this.order = order;
	}

	public UserOrders() {
		super();
	}
	
	
	
}

映射文件UserOrdersMapper.xml配置。注意配置文件，这里是因为发生了一个问题。如果配置文件仍然使用原来数据库的字段，就会使orders的对象填充user的ID，这是错误。我把数据库user 的ID改为uid,orders的id改为oid。然后就正确填充了。也可能是我配置错误。如果你懂的话，请留言给我

<?xml version="1.0" encoding="UTF-8" ?>
<!-- 注意不要弄错了，配置文件是config，这个是映射mapper -->
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.test.mapper.UserOrdersMapper">

<resultMap type="com.test.domain.UserOrders" id="UOlist">
<!-- user class <==> user table  -->
<id property="id" column="uid" />
<result property="username" column="username"/>
<association property="order" javaType="com.test.domain.orders" >
<!-- orders table -->
<id property="id" column="oid"/>
<result property="userid" column="user_id"/>

</association>

</resultMap>
<!-- 查询用户下的订单 -->
<select id="queryUserOrders" resultMap="UOlist">
select 
u.uid,
u.username,
o.oid
from `user` as u
LEFT JOIN orders as o on u.uid = o.user_id
</select>

</mapper>

测试

package com.test.test;

import java.io.IOException;
import java.util.List;

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 org.junit.Test;

import com.test.domain.UserOrders;
import com.test.mapper.UserOrdersMapper;

public class testUserOrders {

	
	public static SqlSession sqlSession;
	public static UserOrdersMapper mapper;
	public static void init() throws IOException {
		SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
		SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(Resources.getResourceAsStream("sqlMapConfig.xml"));
		
		sqlSession = sqlSessionFactory.openSession();
		
		mapper = sqlSession.getMapper(UserOrdersMapper.class);
		System.out.println("testVo init ");
	}
	
	@Test
	public void testQueryUserOrdersList() throws Exception {
		init();
		
		
		 List<UserOrders> list = mapper.queryUserOrders();
		 for (UserOrders userOrders : list) {
			System.out.println(userOrders + " order:" + userOrders.getOrder());
			//System.out.println("++++++);
		}
	}
}

二、一对多

仍然是前面的例子，改变是：在userorders类中增加了list<orders>类型的属性。并设置get，set方法。修改UserOrdersMapper,增加新方法。并对应在UserOrdersMap.xml中增加对应新方法的select标签。
注意：类必须要有一个无参构造函数。


<resultMap type="com.test.domain.UserOrders" id="UOlist2">
<id column="id" property="uid"/>
<result column="username" property="username"/>
<!-- 
与映射的类中对应的list相同，
property是对应类中的属性名
javatype填写对应的类型，本次是list。如果是数组的话应填array
ofType是指list存的类型，本次是orders

-->
<collection property="listorders" javaType="list" ofType="orders">
<id column="oid" property="id"/>
<result column="user_id" property="userid"/>
</collection>
</resultMap>

<select id="queryUserOrders2" resultMap="UOlist2">
select 
u.uid,
u.username,
o.oid
from `user` as u
LEFT JOIN orders as o on u.uid = o.user_id

</select>

【Mybatis】一对一与一对多的关联查询

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