Well, you can sum something and then use the date to divide by. Makes no sense at all.
You can add all the downloaded data where the date is less than one week. You did not show us your table info. If you have a table that tracks downloads, and the date downloaded is kept in the table, you can just get that total with SELECT SUM(downloaded) WHERE download_date >= DATE(NOW()) - INTERVAL 7 DAY; Or something loosely.
But, if you just add the total downloaded each download, you have no way to track it by time at all.
How does your table keep track of downloads? Normally, you would have a table that tracks what the user downloaded, the date/time it was downloaded and the user_id to know who downloaded it. Then, you can do reports based on date and get totals with a simple query. If you want to just keep the running total for a week, you would need to keep the total for each day and then sum all the days. Once a new day comes along, the 7 day total would be accurate back to 7 days. Either way would work.
Not sure if that helps, but, hope so…