I think your brand/date and reverse date/brand index is a great idea for this and all sorts of other applications! I hadn't thought to set them up quite that way in this case.
in reply to Re^5: find difference in dates/items in same column
in thread find difference in dates/items in same column
To maintain the rank would mean searching backward through the table to find the last 'brand' entry and its corresponding rank. I'll investgate triggers and think about how to do it when the new records are being inserted rather than after the fact.
Thanks for all the help! it has been great... Apologies for contaminating a perl forum with what turned out to be a SQL question!