What if there is more than one 'Period Key' for a (Support_Start_Date, Support_End_Date) pair?
Maybe you're looking for something like this? It returns multiple rows if the (Support_Start_Date, Support_End_Date) pair spans multiple 'Period Key'
select i.inv_Crd_No,
pr.Year_Num as Start_Year,
pr.Month_Num as Start_Month,
pr.Period_Key as Supp_start_key,
i.Support_Start_Date,
i.Support_End_Date
from invoice_Credit_Fact i
left join Period_Ref pr ON i.Support_Start_Date >= pr.[Start_Date]
and i.Support_End_Date = pr.End_Date
where i.inv_Crd_No iN ('SiN020693','SiN020685')
order by i.inv_Crd_No, i.Support_Start_Date;
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