UPDATE using previously updated value - sql-server-2008

Question

I'm trying to update records using previously updated value in a single query run. It's a running total case, but it's an UPDATE query not a SELECT query.
Table Inventory (Simplified)
Id Qty RunningQty ItShouldBeUpdatedTo InsteadItsUpdatedTo
1 200 0 200 200
2 300 0 500 300
3 400 0 900 400
4 100 0 1000 100
My current query is something like this
UPDATE Inventory
SET RunningQty = ISNULL(A.RunningQuantity, 0) + Quantity
FROM Inventory I
OUTER APPLY
(
-- Take previous row RunningQty
SELECT TOP 1 RunningQty
FROM Inventory
WHERE Id < I.Id
ORDER BY Id DESC
) A
It seems to update next row, sql server not using previously updated value.
Note: It will be quite large table so i calculate it using previous row running quantity and add the value with current row quantity, instead of calculating it from the first row.
What's the correct way to do this?
Thanks in advance.

Answer 1

Let me preface this by saying, this will be significantly easier to do in SQL Server 2012. In that version you will be able to use something like this:
update i1
set i1.runningqty = i2.RunningQty
from inventory i1
inner join
(
select id, SUM(qty) OVER(ORDER BY id ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) RunningQty
from inventory
) i2
on i1.id = i2.id;
See SQL Fiddle with Demo
But it is not as easy in SQL Server 2008, however you should be able to use something like this:
declare #total int
set #total = 0
update inventory
set runningqty = #total,
#total = #total + qty;
See SQL Fiddle with Demo.
There are other methods including using a cursor to perform this in sql server 2008.