<select id="getOrderItemByOrderNoAndUserId" resultMap="BaseResultMap" parameterType="map">
   select <include refid="Base_Column_List"/>
   from mall_order_item
   where user_id = #{userId} and order_no = #{orderNo}
</select>

List<OrderItem> orderItemList = this.orderItemMapper.getOrderItemByOrderNoAndUserId(orderNo, userId);

因为orderNo是唯一的，这样获取的就是唯一的OrderItem对象，为什么要用orderItemList集合去接收查询到的结果呢？