計算累計差

思路:先將需要進行累積的值變成負值，再進行累加，即得到累計差

with x as

(select rownum as seq,a.*

from (select 編號,項目,金額 from detail order by 編號) a)

select 編號,

          項目,

          金額，

          sum(case when seq=1 then 金額 else -金額 end) over(order by seq) as 餘額

from x;