-- Demonstrate math
declare @date date = '2016-03-31';
select @date CurrentDate, DATEADD(month, -1, @date) ReportDate, ReportYear = YEAR(DATEADD(month, -1, @date)), ReportMonth = Month(DATEADD(month, -1, @date));

-- Here is a query
SELECT s.SalesID
     , s.StoreID
     , s.[Year]
     , s.[Month]
     , s.Sales
FROM
  dbo.Sales s
WHERE
  s.[Year] = YEAR(DATEADD(month, -1, GETDATE()))
  AND s.[Month] = Month(DATEADD(month, -1, GETDATE()));