Best Practices: GROUP BY Clause

I was wondering what the best way to write a GROUP BY clause when there are many (and time consuming) operations in the fields by grouped.

Fictious example:

SELECT DeptNo, AVG(Salary) FROM Department GROUP BY DeptNo;

This will give me the average salary per department. Let's say, however that
I had 10-15 fields being returned (along with the AVG(Salary)) and some fields even had operations being performed on them. Is it better to create a temporary table to calculate the sum per department (or a VIEW) and then
perform a JOIN with the rest of the data?

Fictious example:

SELECT DATENAME(y, StartDate), DATENAME(m, StartDate), DATEPART(d, StartDate), SUBSTR(DeptName, 1, 10), SomeFunction(SomeField), SomeFunction(SomeField), AVG(Salary)
GROUP BY DATENAME(y, StartDate), DATENAME(m, StartDate), DATEPART(d, StartDate), SUBSTR(DeptName, 1, 10), SomeFunction(SomeField), SomeFunction(SomeField);

Am I better off writing my query this way or using a JOIN on some temporary table or view?

Thanks