Forcing rows to appear despite data missing - sql

Question

I have a large data set (called input below) which contains a variety of information such as sales dates, transaction dates, payments, sales.
The user can produce a report by year, quarter or month to show the amount of payment in a particular month/quarter/year of a certain sales year. So you could have a payment 5 years after the initial sale, i.e. a payment in 2016 relating to the sales year of 2011.
The user can decide whether they want to have these payment periods by month/quarter/year at the beginning of the code through the use of macro variables (i.e. %let ReportBasis = Year) and ReportBasis can be called on through the rest of the code without manual adjustments.
The report is produced using:
proc sql;
create table report as
select sales_year, &ReportBasis, Sum(Sales) as Sales
from input
group by sales_year, &ReportBasis;
quit;
Now the issue I am having is that if there is no payment in a particular period for all sales years in question, then there is no row for that period. This produces a layout problem.
I require a solution to this which can be dynamic due to the nature of the macro variable (changing from year to month to quarter).
So I have this (example):
2011 Month 11 100
2011 Month 12 250
2011 Month 13 85
2011 Month 15 90
2011 Month 16 300
But I require this:
2011 Month 11 100
2011 Month 12 250
2011 Month 13 85
2011 Month 14 0
2011 Month 15 90
2011 Month 16 300
where there is no actual payment in month 14 in all of my data (even other years 2012, 2013 etc.), so it doesn't show up in my first table, but the second table still cleverly knows to include it.
Thanks in advance.

Answer 1

Here is one solution that assumes that all years and reports are represented in input, although not all combinations:
create table report as
select sy.sales_year, r.rb, Sum(i.Sales) as Sales
from (select distinct sales_year from input) sy cross join
(select distinct &ReportBasis as rb from input) r left join
input i
on i.sales_year = sy.sales_year and i.&ReportBasis = r.&ReportBasis
group by sy.sales_year, r.rb;