select -- Key fields
       D.Merchant_ID, 'YYYYMM' as Billing_Period, D.TxnType, ...
       -- Statistics fields
       sum(D.Amount) as TxnTotal, count(*) as TxnCount,
       ...
into TxnSumHist_YYYYMM S
from TxnDtls D
where D.TxnDate between ... and ...
group by D.Merchant_ID, D.TxnType, ...