Multicondition Subquery Max Date and Item Number
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WITH MaxSalesDatesPerCustomer
AS
(
SELECT CustomerID, MAX(SalesDate) AS MaxDate
FROM dbo.PurchaseTransactions
GROUP BY CustomerID
)
SELECT MS.CustomerID, MS.MaxDate AS SalesDate, MAX(OrderNumber /*???*/) AS LargestOrderNumberForMaxDatePerCustomer
FROM MaxSalesDatesPerCustomer AS MS
     INNER JOIN
     dbo.PurchaseTransactions AS PT
     ON PT.CustomerID = MS.CustomerID
WHERE MS.MaxDate = PT.SalesDate
GROUP BY MS.CustomerID, MS.MaxDate