What I would like to query is, for example: For each of these rows, how many purchases were made by the same account in the previous 10 days, and what is the sum of purchases for that account over the previous 10 days.
IF there were exactly one row for each account for each day on the calendar, it would be easy to use SUM(purchase) over (PARTITION BY Acct_Nbr ORDER BY Date ROWS BETWEEN 10 PRECEDING AND 1 PRECEDING). However, most days there are no purchases for any given account. Some days there are more than one purchases. A one-record-per-day table would be vastly larger than the sample here.