with t(id, n) as 
(
    select 1, 1 from dual
    union all select 2, 2 from dual 
    union all select 3, -5 from dual
    union all select 4, 6 from dual
)
select * from t
model 
    dimension by (id)
    measures (n, 1 s)
    rules
    (
        s[1] = n[1],
        s[id>1] = case when s[CV()-1] + n[CV()] > 0 then  s[CV()-1] + n[CV()] else 0 end
    );

        ID          N          S
---------- ---------- ----------
         1          1          1
         2          2          3
         3         -5          0
         4          6          6

with t(id, n) as 
(
    select 1, 1 from dual
    union all select 2, 2 from dual 
    union all select 3, -5 from dual
    union all select 4, 6 from dual
),
r(id, n, s) as
(
select id, n, case when n > 0 then n else 0 end from t where id = 1
union all
select t.id, t.n, case when r.s+t.n > 0 then r.s+t.n else 0 end from t, r where r.id = t.id - 1 
)
select * from r