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Combining Window Functions and GROUP BY

Andy Brownsword aggregates some data:

We revisited window functions last week for T-SQL Tuesday. As we’re in that area there’s another example I thought was worth exploring. Can we group data whilst applying window functions in the same query?

Andy comes up with a final query that works perfectly fine, but there’s actually an easier answer in terms of code readability: the DISTINCT operator.

SELECT DISTINCT FinancialQuarter, QuarterAvg = AVG(SalesValue) OVER (PARTITION BY FinancialQuarter), YearAvg = AVG(SalesValue) OVER (PARTITION BY FinancialYear)
FROM #MonthlySales;

The FinancialQuarter column is unique so we can perform the window operation for averaging sales value over financial quarter and then by financial year. To remove the “duplicate” rows, we run DISTINCT and get the same results.

That said, the execution plan for this is a little more complex, as we have to go through a lazy spool on two separate occasions rather than the one that Andy’s solution comes up with. For sufficiently large datasets, that could make a difference, so as usual, choose the option that works better for your situation.