开窗函数 累加计算

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

sum(sum(pur_comp_cnt)) over (partition by pur_id order by pur_comp_create_month rows between unbounded preceding and current row) pur_comp_cnt