Without DDL I can't write the query...
Calculate a row_number over the partition and order that you want for the sum. Do that in a CTE. Then join that to itself on the partition columns and row_number of one = (row number of the other) -1. Then you'll have the two sales as columns and you can trivially add them together.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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