13.高级映射（多对多查询）

1.需求

查询用户及用户购买的商品信息

2.sql

查询主表：用户表
关联表：由于用户和商品没有直接关联，通过订单和订单明细进行关联。
orders、orderdedail、items

SELECT orders.* ,
    user.username,
    user.sex,
    user.address,
    orderdetail.id orderdetail_id,   
    orderdetail.items_id,
    orderdetail.items_num,
    orderdetail.orders_id,
    items.name items_name,
    items.pic items_pic,
    items.price items_price

 FROM orders,user,orderdetail,items
 WHERE orders.user_id=user.id 
       and orderdetail.orders_id=orders.id 
       AND orderdetail.items_id=items.id

3.映射思路

将用户信息映射到user中
在user类中添加定单列表属性List orderslist,将用户创建的订单映射到orderslist
在Orders中添加订单明细列表属性List orderdetials
在OrderDetail中添加Items属性，将订单明细所对应的商品映射到Items

4.mapper.xml

    
        SELECT orders.* ,
                user.username,
                user.sex,
                user.address,
                orderdetail.id orderdetail_id,
                orderdetail.items_id,
                orderdetail.items_num,
                orderdetail.orders_id,
                items.name items_name,
                items.pic items_pic,
                items.price items_price

        FROM orders,user,orderdetail,items
        WHERE orders.user_id=user.id
              and orderdetail.orders_id=orders.id
              AND orderdetail.items_id=items.id
    

5.定义resultMap

在User中添加ordersList属性

 public class User {
    private int id;
    private String username;
    private Date birthday;
    private String sex;
    private String address;

    //多对多映射，一个用户多个订单
    private List ordersList;

在OrderDetail中添加items属性

public class OrderDetail {
 private int id;
 private int ordersId;
 private  int itemsId;
 private int itemsNum;

 //多对多映射，一个明细对应一个商品
    private Items items;

6.OrderCustomMapper.java

    //查询用户购买的商品信息
    public List findUserAndItemsResultMap() throws Exception;

7.测试

 @Test
    public void findUserAndItemsResultMap() throws Exception {
        SqlSession sqlSession=sqlSessionFactory.openSession();
        //创建代理对象
        OrderCustomMapper orderCustomMapper= sqlSession.getMapper(OrderCustomMapper.class);
        //调用mapper的方法
        List list =orderCustomMapper.findUserAndItemsResultMap();
        System.out.println(list);
        sqlSession.close();
    }

8.总结