These built-in T-SQL functions provide the same functionality as the query above but with less complicated coding and more efficiency.

what-is-sql-cross-apply-11.sql

SELECT 
    SalesOrderDetailID,
    UnitPrice,
    CarrierTrackingNumber,
    RunningTotal = SUM (UnitPrice) OVER (
        PARTITION BY CarrierTrackingNumber ORDER BY SalesOrderDetailID),
    RowCnt= COUNT (*) OVER (
        PARTITION BY CarrierTrackingNumber ORDER BY SalesOrderDetailID)
FROM Sales.SalesOrderDetail o
WHERE o.CarrierTrackingNumber IS NOT NULL
ORDER BY CarrierTrackingNumber ASC;