As you can see from people's (lack of) response, if you want discussion, you might be better off giving your own thoughts to kick the discussion into gear. That said, my guess is that no, the order should not matter. If you group SUM(sales) by "year, region", you are asking to see totals for each year and totals for each region and if you group them by "region, year", you are still asking for totals for each region and for each year (in other words, the same two totals). Is that what you'd expect? What about it merits discussion?