WITH #help1 as (
select *
,ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) AS Row#
, case when [Masked] is null then 0 else 1 end as ChangeIndicator
from [dbo].[table1]
)
, #help2 as (
select *, Sum(ChangeIndicator) over (ORDER BY Row#) RowGroup from #help1
)
select
case when [Masked] is not null then [Masked]
else first_value([Masked]) over (partition by RowGroup ORDER BY Row#)
end UnitsBalanceFillDown
from #help2
