Mybatis的一对多和多对一关系

1. 一对多

–注意事项，使用左外连接而非内连接
SELECT c., o.
from t_customer c left outer join t_order o on c.customer_id = o.cid
where c.customer_id = 4;

SELECT c., o.
from t_customer c inner join t_order o on c.customer_id = o.cid
where c.customer_id = 4;

2. 多对一
   
   
   
   

   为什么多对一可以使用内连接？？？
   select o., c.
   from t_order o inner join t_customer c on o.cid = c.customer_id
   where o.order_id = #{orderId,jdbcType=INTEGER}

3. 多对多

SELECT n.news_id, n.title, c.category_id, c.category_name
FROM t_news n left outer join t_news_category nc on n.news_id = nc.nid left outer join t_category c on nc.cid = c.category_id
where n.news_id = #{newsId,jdbcType=INTEGER}

4. 新闻添加或删除类别

5. 新闻删除

6. 作业
   多对多关系删除