I understand the logic behind listing #3, but I was surprised to see, and uncertain on how it works, that it is not necessary to use another GROUP BY clause in the subquery:

SELECT Outer_H.[CustomerID]

, SUM(Outer_H.[SubTotal]) AS TotalPurchase

, SUM(Outer_H.[SubTotal]) * .10 AS Rebate

FROM [Sales].[SalesOrderHeader] AS Outer_H

WHERE YEAR(Outer_H.[OrderDate]) = '2008'

GROUP BY Outer_H.[CustomerID]

HAVING (SELECT SUM(Inner_H.[SubTotal]) FROM [Sales].[SalesOrderHeader] AS Inner_H

WHERE Inner_H.[CustomerID] = Outer_H.[CustomerID]

AND YEAR(Inner_H.[OrderDate]) = '2008' GROUP BY CLAUSE NOT REQUIRED HERE) > 150000

ORDER BY Rebate DESC;

How is SQL able to execute the aggregate SUM() function in the inner query w/out a GROUP BY clause applied? Does it automatically apply the GROUP BY clause from the outer query to the inner query?

Thanks.