Need help with complicated SQL statement - Mailing list pgsql-general

Please consider the following statement (it becomes
obvious if you remember the important thing about the
table is that it has columns for each of stock_id,
price_date, and price).

(SELECT * FROM stockprices WHERE stock_id = 1 ORDER BY
price_date DESC LIMIT 1)
UNION
(SELECT * FROM (SELECT * FROM stockprices WHERE
stock_id = 1 ORDER BY price_date DESC LIMIT 22) AS T2
ORDER BY T2.price_date ASC LIMIT 1)
UNION
(SELECT * FROM (SELECT * FROM stockprices WHERE
stock_id = 1 ORDER BY price_date DESC LIMIT 66) AS T3
ORDER BY T3.price_date ASC LIMIT 1)
UNION
(SELECT * FROM (SELECT * FROM stockprices WHERE
stock_id = 1 ORDER BY price_date DESC LIMIT 132) AS T4
ORDER BY T4.price_date ASC LIMIT 1)
UNION
(SELECT * FROM (SELECT * FROM stockprices WHERE
stock_id = 1 ORDER BY price_date DESC LIMIT 264) AS T5
ORDER BY T5.price_date ASC LIMIT 1);

This statement works flawlessly, and is blindingly
fast relative to everything else I have tried.  But I
am stuck.

First, while this statement gets me the correct data,
I need to obtain a single record with stock_id,
current price (that obtained from the first select
statement in the union, and each of the prices
returned by the subsequent select statements as a the
current price minus the price at the previous date,
and the result divided by the price at the previous
date, expressed as a percentage.  I do not yet know
how to do this using SQL (it would be trivial if I
exported the data to Java or C++ - but it isn't clear
how to do it within SQL).

To make things more difficult, suppose I have another
select statement that returns a set of stock_ids.  How
do I apply the SQL logic I require to only those
stocks in the set returned by a statement like SELECT
stock_id FROM someTable WHERE ...  The result of this
extension would be that I have one record for each
stock in the selected set of stocks.

I do NOT want to have to recompute the set of stocks
for each of the select statements in the above union
(since that would be a waste because the resulting set
of stocks would always be the same for the given
criteria).  Nor do I want to apply the SQL logic I
need for the prices to all the stocks in the database.
 There could be thousands, or even tens of thousands,
of stocks represented in the database and I'd need the
gain/loss logic only for a few dozen at any given
time!

How do I make the two extensions I require?

I expect the SQL I get to be eventually placed in a
stored procedure, which may then be used to construct
a view, but that is the easy part.

Maybe I have been staring at this for too long to see
the obvious solution, but I am exhausted and am not
seeing the next step.  If there IS an obvious next
step, please at least give me a hint.

Thanks

Ted