This question is quite hard to answer without a sample of data to work against. I say this because it depends on what your definition of "all sales are in the database" is (data for every day up to today during the year, data for a full calendar year, etc ?? ) and how the data that is missing is stored (is it stored as nulls, is it stored as 0s ??).
If you could post a sample dataset or two--one where we would use sales_this_year and one where sales_this_year is incomplete but sales_last_year is complete--somebody may be able to help out.