使用java+mybaits实现订单表和商品表的一对多查询并打印，用户表和订单表的一对一查询并打印

假设订单表为`orders`，商品表为`items`，用户表为`users`，其中订单表和商品表为一对多关系，用户表和订单表为一对一关系。以下是使用Java和MyBatis实现的示例代码：

1. 首先，我们需要创建对应的Java对象，以及对应的Mapper接口：

Orders.java

public class Orders {
    private int id;
    private String orderNo;
    private int userId;
    private List<Items> itemsList;
    // 省略getter和setter方法
}

Items.java

public class Items {
    private int id;
    private String itemName;
    private int orderId;
    // 省略getter和setter方法
}

Users.java

public class Users {
    private int id;
    private String username;
    private Orders orders;
    // 省略getter和setter方法
}

OrdersMapper.java

public interface OrdersMapper {
    List<Orders> findOrdersWithItems();
    Orders findOrderWithUserById(int id);
}

2. 然后，在MyBatis的配置文件中，我们需要定义对应的SQL语句：

ordersMapper.xml

<mapper namespace="com.example.mapper.OrdersMapper">
    <resultMap id="ordersMap" type="com.example.model.Orders">
        <id property="id" column="id"/>
        <result property="orderNo" column="order_no"/>
        <result property="userId" column="user_id"/>
        <collection property="itemsList" ofType="com.example.model.Items">
            <id property="id" column="id"/>
            <result property="itemName" column="item_name"/>
            <result property="orderId" column="order_id"/>
        </collection>
    </resultMap>

    <select id="findOrdersWithItems" resultMap="ordersMap">
        SELECT o.id, o.order_no, o.user_id,
            i.id, i.item_name, i.order_id
        FROM orders o
            LEFT JOIN items i ON o.id = i.order_id
    </select>

    <select id="findOrderWithUserById" resultType="com.example.model.Orders">
        SELECT o.id, o.order_no, o.user_id,
            u.id as user_id, u.username
        FROM orders o
            LEFT JOIN users u ON o.user_id = u.id
        WHERE o.id = #{id}
    </select>
</mapper>

3. 最后，我们在Java代码中使用Mapper接口来查询数据并打印：

public class Main {
    public static void main(String[] args) {
        SqlSession sqlSession = MyBatisUtil.getSqlSession();
        OrdersMapper ordersMapper = sqlSession.getMapper(OrdersMapper.class);

        // 一对多查询
        List<Orders> ordersList = ordersMapper.findOrdersWithItems();
        for (Orders orders : ordersList) {
            System.out.println("订单信息：");
            System.out.println("订单号：" + orders.getOrderNo());
            System.out.println("用户ID：" + orders.getUserId());

            List<Items> itemsList = orders.getItemsList();
            for (Items items : itemsList) {
                System.out.println("商品信息：");
                System.out.println("商品名称：" + items.getItemName());
            }
        }

        // 一对一查询
        Orders order = ordersMapper.findOrderWithUserById(1);
        System.out.println("订单信息：");
        System.out.println("订单号：" + order.getOrderNo());
        System.out.println("用户信息：");
        Users user = order.getUser();
        System.out.println("用户名：" + user.getUsername());
    }
}

这样，就可以实现订单表和商品表的一对多查询并打印，用户表和订单表的一对一查询并打印。