废话不多说,直接上代码

(重点)UserMapper.xml文件中关键映射代码,注意不能省略正常匹配的键,还有collection中使用ofType来通知类型

<resultMap id="queryUserWithOrders" type="User">
        <result column="user_id" property="id"/>
        <result column="username" property="username"/>
        <collection property="orders" ofType="Orders">
            <result column="number" property="number"/>
            <result column="id" property="id"/>
        </collection>
    </resultMap>
    <select id="queryUserWithOrders" resultMap="queryUserWithOrders">
      select
      u.username,
      o.*
      from `user` u
      LEFT JOIN orders o
      on u.id = o.user_id
    </select>

UserMapper.java接口代码

public interface UserMapper {

    User queryUserById(Integer id);

    List<User> queryUserByUsername(String name);


    public List<User> queryUserWithOrders();
}

pojo类User.java中代码:(增加一个orders并生成get和set方法,顺便重写了toString以便打印)

private List<Orders> orders;

    public List<Orders> getOrders() {
        return orders;
    }

    public void setOrders(List<Orders> orders) {
        this.orders = orders;
    }