SQL Server : calculate monthly total sales incl empty months

Question

I\'m trying to calculate the total sales of a product in a month, but I would like it to include any \"empty\" months (with no sales) and only select the latest 12 months.

Answer 1

Try:

;with CTE as 
(select 0 months_ago union all 
 select months_ago - 1 months_ago from CTE where months_ago > -11),
month_list as 
(select dateadd(MONTH, 
                months_ago, 
                dateadd(DAY, 
                        1-datepart(DAY,getdate()),
                        cast(GETDATE() as DATE))) month_start 
 from cte)
SELECT YEAR(ml.start_date) as 'Year', 
       MONTH(ml.start_date) as 'Month', 
       sum(Amount) as 'Units sold',[ProductNo]
FROM month_list ml
left join [Order] o 
       on o.OrderDate >= ml.start_date and 
          o.OrderDate < dateadd(MONTH, 1, ml.start_date)
left join [OrderLine] ol 
       on ol.OrderNo = o.OrderNo and ProductNo = @ProductNo
Group by ProductNo, YEAR(ml.start_date), Month(ml.start_date)
Order by ProductNo, YEAR(ml.start_date), Month(ml.start_date)