How do I sort by a rolled up group? - sorting

Question

Let's say I have a table in SQL Server 2008 R2 called Purchase with two columns: Purchaser and Expenditure.
Let's say that the table has the following rows:
Purchaser Expenditure
--------- -----------
Alex 200
Alex 300
Alex 500
Bob 300
Bob 400
Charlie 200
Charlie 600
Derek 100
Derek 300
Now I have this query:
SELECT Purchaser, Expenditure, SUM(Expenditure) AS SumExpenditure FROM Purchase GROUP BY Purchaser, Expenditure WITH ROLLUP
This returns the following:
Purchaser Expenditure SumExpenditure
--------- ----------- --------------
Alex 200 200
Alex 300 300
Alex 500 500
--------------------------------
Alex NULL 1000
--------------------------------
Bob 300 300
Bob 400 400
--------------------------------
Bob NULL 700
--------------------------------
Charlie 200 200
Charlie 600 600
--------------------------------
Charlie NULL 800
--------------------------------
Derek 100 100
Derek 300 300
--------------------------------
Derek NULL 400
--------------------------------
(Lines added to emphasise the rolled up amounts.)
What I would like would be to be able to sort the groups by the grouped amounts so that I end up with a result set like this:
Purchaser Expenditure SumExpenditure
--------- ----------- --------------
Derek 100 100
Derek 300 300
--------------------------------
Derek NULL 400
--------------------------------
Bob 300 300
Bob 400 400
--------------------------------
Bob NULL 700
--------------------------------
Charlie 200 200
Charlie 600 600
--------------------------------
Charlie NULL 800
--------------------------------
Alex 200 200
Alex 300 300
Alex 500 500
--------------------------------
Alex NULL 1000
--------------------------------
In other words, I am sorting the groups, using the 400, 700, 800 and 1000 in the group rows by ascending order.
Can anyone suggest what query would return this result set?

Answer 1

;WITH x AS
(
SELECT Purchaser, Expenditure, s = SUM(Expenditure)
FROM dbo.Purchase
GROUP BY Purchaser, Expenditure WITH ROLLUP
),
y AS
(
SELECT Purchaser, s FROM x
WHERE Expenditure IS NULL
AND Purchaser IS NOT NULL
),
z AS
(
SELECT Purchaser, s, rn = ROW_NUMBER() OVER (ORDER BY s)
FROM y
)
SELECT x.Purchaser, x.Expenditure, x.s FROM x
INNER JOIN z ON x.Purchaser = z.Purchaser
ORDER BY z.rn, CASE WHEN z.s IS NULL THEN 2 ELSE 1 END;