如何在Oracle中为具有先前的第一个非空值的空值分配等级

Question

I need to assign a rank to some null values over ordered rows.
My query is like this :

with sub as 
(
 select 10 as id, 1 as inx,2 as num from dual
  union all
 select 10 as id, 2 as inx,null as num from dual
  union all
 select 10 as id, 3 as inx,8 as num from dual
  union all
 select 10 as id, 4 as inx,null as num from dual
)
select *
  from sub order by inx

and result set is like this :

id  inx  num
---------- 
10  1    2
10  2    null
10  3    8
10  4    null

i'm tring to set null values with previous first non-null value
for example : num null value should be "2" where inx = 2
and num null value should be "8" where inx = 4 and so on.

thx for any idea..

Answer 1

If you know that the values are increasing, you can just use max() :

select id, inx, max(num) over (partition by id order by inx) as num

If they are not increasing and multiple nulls never appear in a sequence, you can use lag() :

select id, inx,
       (case when num is null
             then lag(num) over (partition by id order by inx)
             else num
        end)as null;

If nulls do appear in a sequence, you can use the ignore nulls option to lag() :

select id, inx,
       (case when num is null
             then lag(num ignore nulls) over (partition by id order by inx)
             else num
        end)as null