Ajax,
Did the code you suggested work on your machine?  I couldn't get it to 
work on mine without adding some GROUP BY statements:

SELECT A.pin, A.failed,b.attempts,(cast(A.failed as float)/cast(B.attempts 
as float)*100) as failrate
FROM
(SELECT pin,count(pin)as failed from eventlog where pass_fail = 0 GROUP BY 
pin) A,
(SELECT pin,count(pin) as attempts  from eventlog GROUP BY pin) B
WHERE
A.pin = B.pin

As an aside, I tested our 2 suggested methods on a fictional dataset (4000 
rows) to see the performance difference.  Interestingly the difference 
wasn't anywhere near as great as I expected - at around 15%.

cheers,
andy