Writing

SUM(Quantity) OVER (ORDER BY EffectiveDate RANGE UNBOUNDED PRECEDING) AS TQuantity

does the trick. I will have to learn more about this feature as I currently have the bad feeling of programming...

I am getting deadlocks (SqlException 1205). I understand that my test case is highly hypothetical but I wanted to have deadlocks for sure.

I am running the threads on a client machine and each thread uses its own SqlConnection component. As far as I know, this is equivalent of having several computers running...

Tricking the T-SQL compiler is a clever solution. Better, performance penalty is insignificant. Thanks a lot!

Using EXEC works well in a standard script but calling it from within a function gives the error "Invalid use of a side-effecting operator 'INSERT EXEC' within a function".