mybatis association与collection的区别

<resultMap type="cn.itcast.mybatis.po.Orders" id="OrdersUserResultMap">
        <!-- 配置映射的订单信息 -->
        <id column="id" property="id" />
        <result column="user_id" property="userId" />
        <result column="number" property="number" />
        <result column="createtime" property="createTime" />
        <result column="note" property="note" />
        <!-- 配置关联的用户信息 -->

        <association property="user" javaType="cn.itcast.mybatis.po.User">
            <id column="user_id" property="id" />
            <result column="username" property="username" />
            <result column="sex" property="sex" />
            <result column="address" property="address" />
        </association>

    </resultMap>

    <resultMap type="cn.itcast.mybatis.po.Orders" id="OrdersAndOrdersDetail" extends="OrdersUserResultMap">

        

        <!-- 配置订单详细信息 一个订单关联查出了多条明细,要使用collection进行映射 collection:对关联查到的多条记录映射到集合对象中 
            property:将关联查询到多条记录映射到对象的某个属性中 ofType 指定映射集合属性的的pojo对象的类型 -->
        <collection property="orderdetails" ofType="cn.itcast.mybatis.po.Orderdetail">
             <!-- id：订单明细惟 一标识
             property:要将订单明细的惟 一标识 映射到cn.itcast.mybatis.po.Orderdetail的哪一个属性
               -->
             <id column="orderdetail_id" property="id"/>
             <result column="items_id" property="itemsId"/>
             <result column="items_num" property="itemsNum"/>
             <result column="orders_id" property="ordersId"/>
        
        </collection>

    </resultMap>