Year over year monthly sales - tsql

Question

I am using SQL Server 2008 R2. Here is the query I have that returns monthly sales totals by zip code, per store.
select
left(a.Zip, 5) as ZipCode,
s.Store,
datename(month,s.MovementDate) as TheMonth,
datepart(year,s.MovementDate) as TheYear,
datepart(mm,s.MovementDate) as MonthNum,
sum(s.Dollars) as Sales,
count(*) as [TxnCount],
count(distinct s.AccountNumber) as NumOfAccounts
from
dbo.DailySales s
inner join
dbo.Accounts a on a.AccountNumber = s.AccountNumber
where
s.SaleType = 3
and s.MovementDate > '1/1/2016'
and isnull(a.Zip, '') <> ''
group by
left(a.Zip, 5),
s.Store,
datename(month, s.MovementDate),
datepart(year, s.MovementDate),
datepart(mm, s.MovementDate)
Now I'd like to add columns that compare sales, TxnCount, and NumOfAccounts to the same month the previous year for each zip code and store. I also would like each zip code/store combo to have a record for every month in the range; so zeros if null.
I do have a calendar table that I tried to use to get all months, but I ran into problems because of my "where" statements.
I know that both of these issues (comparing to previous year and including all dates in a date range) have been asked and answered before, and I've gotten them to work before myself, but this particular one has me running in circles. Any help would be appreciated.
I hope this is clear enough.
Thanks,
Tim

Answer 1

Treat the Query you have above as a data source. Run it as a CTE for the period you want to report, plus the period - 12 months (to get the historic data). (SalesPerMonth)
Then do a query that gets all the months you need from your calendar table as another CTE. This is the reporting months, not the previous year. (MonthsToReport)
Get a list of every valid zip code / Store combo - probably a select distinct from the SalesPerMonth CTE this would give you only combos that have at least one sale in the period (or historical period - you probably also want ones that sold last year, but not this year). Another CTE - StoreZip
Finally, your main query cross joins the StoreZip results with the MonthsToReport - this gives you the one row per StoreZip/Month combos you are looking for. Left join twice to the SalesPerMonth data, once for the month, once for the 1 year previous data. Use ISNULL to change any null records (no data) to zero.
Instead of CTEs, you could also do it as separate queries, storing the results in Temp tables instead. This may work better for large amounts of data.