[PERFORM] Query performance

[Bill]

Actually, I have some queries that are slow, however I was wondering if you could help me write a query that is rather simple, but I, as a true database novice, can't seem to conjure.  So we have stocks, as I have previously said, and I have a huge table which contains all of the opening and closing prices of some stocks from each day.  What I like to do, in English, for each stock in each day is find a ratio: abs(closing-opening)/opening.  Then I would like to average all of the ratios of each day of each individual stock together to find a final ratio for each stock, then I would like to find the highest average, to find the best performing stock.  So what query can I use, and (as is appropriate for this group), how can it be optimized to run the fastest?