with NotNulls    as (select id, category from @SomeTable where category is not null),
     OnlyNulls   as (select id, category from @SomeTable where category is null),
     Combination as (select n.id, 
                            v.category, 
                            row_number() over (partition by n.id order by v.id desc) as rn
                      from OnlyNulls as n
                           inner join NotNulls as v
                           on n.id > v.id),
     Result as (select * from combination where rn = 1)
select t.id,
       t.category,
       coalesce(t.category, t1.category, 0)
  from @SomeTable as t
       left outer join Result as t1
       on t.id = t1.id

select
  t.id,
  t.category,
  coalesce(
    t.category,
    (select top 1 t1.category from @SomeTable t1 where t1.id < t.id and t1.category is not null order by t1.id desc),
    0
  )
   from @SomeTable t

select t.id,
       t.category,
       coalesce(t.category, t1.category, 0)
  from @SomeTable as t
       left outer join (select t2.id, 
                               t5.category
                          from @SomeTable as t2
                               cross apply (select max(t3.id) as id
                                             from @SomeTable as t3
                                            where t3.id <= t2.id 
                                                  and t3.category is not null
                               ) as t4
                               inner join @SomeTable as t5
                               on t4.id = t5.id
       ) as t1
       on t.id = t1.id