creating a SQL Query

Hi friends!
I am facing a difficulty in creating a query in SQL.
I have a table name as master which has fields
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pid | pretailPrice | pourPrice | cat
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1 10 8 1
2 8 7 1
3 7 5 1
------------------------------------------------------------------

I am trying to create a query, which calculates discount between pretailprice and pourprice and the output should give the maximum discount from all the records for a particular category i.e 1.


Please help
Thank you.

SELECT MAX(pourPrice - pretailPrize) as Discount
FROM master
GROUP BY cat

Thanx Kris,
The output comes as the max diffrence,
but we need to calculate the maximum percentage of discount

If we have 3 records for a single category..
---------------------------------------------------------------------------
for eg1 Retail Price is 10, Our Price is 8
equation is 10-8/10 % ---- out put is 20%

for eg2 Retail Price is 8, Our Price is 8
equation is 8-7/7 % ---- out put is 12.5%

for eg2 Retail Price is 7, Our Price is 5
equation is 7-5/7 % ---- out put is 28.57%
---------------------------------------------------------------------------

Now the max is 28.57% correct...I need to prepare a SQL statement which gived the max discounted percentage.
I am Sorry for not mentioning percentage before.
Thank u.

SELECT MAX((1 - (pourPrice / pretailPrize))*100) as Discount
FROM master
GROUP BY cat

Thanx Kris.
Thanks for your help..
I modified your query a bit and got the result..
Thank you.
Thanx a lot for your time.
Thanx again