select
id,
customer,
tickets,
nvl(
sum(tickets) over (
order by id rows between unbounded preceding and 1 preceding
), 0) as offset
from (
select
rownum id,
customer,
tickets
from tickets
)
o anche più brevi (senza introdurre id
s)
select
customer,
tickets,
nvl(
sum(tickets) over (
order by rownum rows between unbounded preceding and 1 preceding
), 0) as offset
from tickets