Advance SQL subquery

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