with t1 as (
select 'a' as tovar,1 as num, 'try2' as try
union all select 'a' as tovar,2 as num, null
union all select 'a' as tovar,3 as num,null 
union all select 'a' as tovar,4 as num,'try1'
union all select 'a' as tovar,5 as num,null 

union all select 'b' as tovar,1 as num, 'try1' as try
union all select 'b' as tovar,2 as num, null
union all select 'b' as tovar,3 as num,'try2'
),
t2 as
(
 select
  tovar, num, try,
  max(str(num, 10) + try) over (partition by tovar order by num rows between unbounded preceding and 1 preceding) as s
 from
  t1
)
select
 tovar, num, try,
 isnull(try, substring(s, 11, cast(0x7fffffff as int))) as try_new
from
 t2;