Inventory difference within a period

[#1]

Could anyone help with this, supposedly, easy exercise? I've got an sql table containing inventory values at certain pointrs of time e.g.:

Date, Value

2009-01-01, 10

2009-06-30, 9

2009-12-31, 12

2010-01-01, 7

and would like to find difference between the last and the first within a period to get:

-1 for 01-06.2009

2 for 01-12.2009

0 for 01-12.2010 (one record is the first and the last at the same time)

Due to the complexity of the rest of the script I need to set it within sql code.

Thanks, Przemek

[disqr_rm]

I am cofused with your sample data and result. Could you please double check and explain little further?

[#3]

Done. It should make sense now.

[#4]

Can anyone help, please?

[#5]

Anyone any idea, please?