Em vez de CURRENT ROW, você pode usar a palavra-chave PRECEDING para somar até a linha anterior.
with data as (
select 1 id, 'A' name, 'fruit' r_group, '2007' year, '04' month, 5 sales from dual union all
select 2 id, 'Z' name, 'fruit' r_group, '2007' year, '04' month, 99 sales from dual union all
select 3 id, 'A' name, 'fruit' r_group, '2008' year, '05' month, 10 sales from dual union all
select 4 id, 'B' name, 'vegetable' r_group, '2008' year, '07' month, 20 sales from dual )
select t.*,
coalesce(sum(sales) over (partition by r_group order by year, month rows between unbounded preceding and 1 preceding),0) opening,
sum(sales) over (partition by r_group order by year, month rows between unbounded preceding and current row) closing
from (
select year, month, r_group, sum(sales) sales
from data
group by year, month, r_group
) t
order by 3,1,2;
year month r_group sales opening closing
---------------------------------------------------
2007 04 fruit 104 0 104
2008 05 fruit 10 104 114
2008 07 vegetable 20 0 20
