SQL - How to Sum every line with record-3

Question

It's dificil ton explain, i want to Sum the 3 last row for every row. An exemple will be better :

ID  VALUE1  RESULT I WANT
===================
1   10   10+5+20 = 35
2   5    5+20+4  = 29
3   20   20+4+50 = 74
4   4    4+50+300 = 354
5   50   50+300+10 = 360
6   300  300+10+15 = 325
7   10   NULL
8   15   NULL

I use Oracle SQL. Thanks

with dummy as 
(
   select 10 as value from dual union all
   select 5 from dual union all
   select 20 from dual union all
   select 4 from dual union all
   select 50 from dual union all
   select 300 from dual union all
   select 10 from dual union ALL
   select 15 from dual 
)
SELECT value, SUM(value) FROM dummy GROUP BY value

i dont have idea to how to process

Answer 1

To get result just for rows having 3 values summed - use conditional Count() Over() analytic function:

WITH      --  S a m p l e    D a t a :
  dummy as 
    ( Select 1 "ID", 10 "VALUE" From Dual Union All
      Select 2,   5 From Dual Union All
      Select 3,  20 From Dual Union All
      Select 4,   4 From Dual Union All
      Select 5,  50 From Dual Union All
      Select 6, 300 From Dual Union All
      Select 7,  10 From Dual Union All
      Select 8,  15 From Dual 
)

--    S Q L :
SELECT ID, VALUE, 
       Case When Count(ID) OVER (ORDER BY ID ROWS BETWEEN CURRENT ROW AND 2 FOLLOWING) = 3
            Then Sum(VALUE) OVER (ORDER BY ID ROWS BETWEEN CURRENT ROW AND 2 FOLLOWING)
      End "TOTAL_3_ROWS"
FROM dummy;

/*    R e s u l t :
ID  VALUE   TOTAL_3_ROWS
--  -----  -------------
 1     10             35
 2      5             29
 3     20             74
 4      4            354
 5     50            360
 6    300            325
 7     10           null
 8     15           null    */