rows between unbounded preceding and current row等，unbounded ，preceding ，following，current row含义详解

unbounded：无界限

preceding：从分区第一行头开始，则为 unbounded。 N为：相对当前行向前的偏移量

following ：与preceding相反,到该分区结束，则为 unbounded。N为：相对当前行向后的偏移量

current row：顾名思义，当前行，偏移量为0

例子：

partition by  order by asc/desc rows between unbounded preceding and current row

partition by order by asc/desc rows between 1 preceding and 1 following