SELECT Question

Hi,
i want to calculate the price difference, change% of 2 price records. Is
there an easy way to do that within one query, rather than writing a
function?

Example:

company, price_date, price, change, change_perc
compA, 20060203,100,,,
compA, 20060202,100,,,
compA, 20060201,100,,,

for one, i would like to calculate for one date the changes (price, and
percentage) to the previous day and update the record.

i also want to be able to run that through the entire history to
re-calculate the changes i.e. not only update one record but update all
of these.

Currently I am doing it with a function but I wonder if there is an
easier and faster way.

Any suggestion or help would be appreciated.

Thanks in advance
Alex