declare @t table (id int, quan int, [week] int)
insert @t select 123, 12, 5 union all select 124, 41, 5 union all select 125, 23, 6
select * from @t

select sum(quan) [quan], [week]
into #gr
from @t
group by [week]

select sum(t2.quan) [quan], t1.[week]
from #gr t1
left join #gr t2 on t1.[week] >= t2.[week]
group by t1.[week]

drop table #gr

declare @t table (id int, quan int, [week] int)
insert @t select 123, 12, 5 union all select 124, 41, 5 union all select 125, 23, 7

select sum(quan) [quan], [week]
into #gr
from @t
group by [week]

select sum(t2.quan) [quan], t1.[week]
into #inc
from #gr t1
left join #gr t2 on t1.[week] >= t2.[week]
group by t1.[week]

;with a as (
  select min([week]) [min_w], max([week]) [max_w] from @t
  union all
  select min_w + 1, max_w from a where min_w < max_w
)
select t.quan, a.min_w [week]
from a
outer apply(select top 1 quan from #inc t where t.[week] <= a.min_w order by t.[week] desc) t

drop table #gr
drop table #inc