SQL Subqueries on each result row

Hello,

I have a table like :

date | value
-------------------------------
2009-09-19 | 1
2009-09-20 | 2
2009-09-21 | 6
2009-09-22 | 9
2009-09-23 | 1

I'd like a request which gives me the sum of each last n days.
For example, if I want the sum of each 3 days, I want this result:

date | sum_value
-------------------------------
2009-09-19 | 1 (sum from 2009-09-17 to 2009-09-19)
2009-09-20 | 3 (sum from 2009-09-18 to 2009-09-20)
2009-09-21 | 9 (sum from 2009-09-19 to 2009-09-21)
2009-09-22 | 17 (sum from 2009-09-20 to 2009-09-22)
2009-09-23 | 16 (sum from 2009-09-21 to 2009-09-23)

I try to make a subquery which is apply on each row of a query, but it
does work.

Has anybody an idea?

Thanks in advance!

Anthony