MyBatis 关联查询的实现：多对多

2个实体：订单、商品，一个订单可以包含多种商品，同时一种商品可以属于多个订单，即多对多。

商品表goods_tb：

订单表order_tb：

no是订单编号，user_id与用户表的id关联。

需要新建一张中间表order_item_tb，引入2个“多”的主键作为外键，把这2个“多”联系起来：

purchase_amount是该种商品的购买数量。

---

使用嵌套结果实现一对多

（一）编写pojo类

package com.chy.pojo;

public class Goods {
    private Integer id;  //商品id
    private String goodsName;  //商品名称
    private float goodsPrice;  //商品单价
    private Integer purchaseAmount;  //购买数量

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getGoodsName() {
        return goodsName;
    }

    public void setGoodsName(String goodsName) {
        this.goodsName = goodsName;
    }

    public float getGoodsPrice() {
        return goodsPrice;
    }

    public void setGoodsPrice(float goodsPrice) {
        this.goodsPrice = goodsPrice;
    }

    public Integer getPurchaseAmount() {
        return purchaseAmount;
    }

    public void setPurchaseAmount(Integer purchaseAmount) {
        this.purchaseAmount = purchaseAmount;
    }

    @Override
    public String toString() {
        return "Goods{" +
                "id=" + id +
                ", goodsName='" + goodsName + '\'' +
                ", goodsPrice=" + goodsPrice +
                ", purchaseAmount=" + purchaseAmount +
                '}';
    }
}

package com.chy.pojo;

import java.util.List;

public class Order {
    private Integer no;
    private Integer userId;
    private List<Goods> goodsList;  //包含的商品

    public Integer getNo() {
        return no;
    }

    public void setNo(Integer no) {
        this.no = no;
    }

    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }

    public List<Goods> getGoodsList() {
        return goodsList;
    }

    public void setGoodsList(List<Goods> goodsList) {
        this.goodsList = goodsList;
    }

    @Override
    public String toString() {
        return "Order{" +
                "no=" + no +
                ", userId=" + userId +
                ", goodsList=" + goodsList +
                '}';
    }
}

在哪个pojo中使用List来关联其他实体，需要根据业务需求来确定。

（二）编写Mapper接口、映射文件

package com.chy.mapper;

import com.chy.pojo.Order;

public interface OrderMapper {
    //根据orderId查询订单信息
    public Order queryOrderByOrderNo(Integer orderNo);
}

<?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.chy.mapper.OrderMapper">
    <select id="queryOrderByOrderNo" parameterType="integer" resultMap="orderResultWithGoods">
        SELECT order_tb.*,goods_tb.*,order_item_tb.purchase_amount
        FROM order_tb,goods_tb,order_item_tb
        WHERE order_tb.no=#{no} AND order_item_tb.order_no=order_tb.no AND goods_tb.id=order_item_tb.goods_id
    </select>
    <resultMap id="orderResultWithGoods" type="order">
        <id property="no" column="no"/>
        <result property="userId" column="user_id"/>
        <collection property="goodsList" ofType="goods">
            <id property="id" column="id"/>
            <result property="goodsName" column="goods_name"/>
            <result property="goodsPrice" column="goods_price"/>
            <result property="purchaseAmount" column="purchase_amount"/>
        </collection>
    </resultMap>
</mapper>

三表联合查询，sql语句很长，如果觉得表名、字段名不好写，可以用as设置表名、字段名的别名。

不是每个pojo类都要写对应的Mapper接口、映射文件，写哪些，看业务需求。

（三）使用

package com.chy.utils;

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;

public class MyBatisUtils {
    private static SqlSessionFactory sqlSessionFactory;

    static {
        try {
            InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

package com.chy.test;

import com.chy.mapper.OrderMapper;
import com.chy.pojo.Order;
import com.chy.utils.MyBatisUtils;
import org.apache.ibatis.session.*;

public class Test {
    public static void main(String[] args) {
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        Order order = mapper.queryOrderByOrderNo(1);
        System.out.println(order);
        sqlSession.close();
    }
}

结果：

Order{no=1, userId=1, goodsList=[Goods{id=1, goodsName='抽纸', goodsPrice=6.5, purchaseAmount=1}, Goods{id=2, goodsName='中华', goodsPrice=80.0, purchaseAmount=2}]}

也可以使用嵌套查询来实现多对多，但嵌套查询缺点较多，不推荐。