Requête récursive Oracle 10g

  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
  )

ou encore plus court (sans introduire id s)

  select 
    customer,
    tickets,
    nvl(
      sum(tickets) over (
        order by rownum rows between unbounded preceding and 1 preceding
      ), 0) as offset
  from tickets