Ordered Update Problems

I€™m trying to follow examples from: http://www.sqlmag.com/Articles/ArticleID/49240/49240.html?Ad=1 for an update statement that needs to run in a specified order.


Here€™s my problem:

I have a table:

TopicActivity
PK €“ DayNum int (this is a date in YYYYDDD format)
PK €“ TopicId int
Visits int
LifetimeVisits int

LifetimeVisits is a new column that I want to calculate the value using the previous Visits column data.

Sample Data:
DayNum TopicId Visits LifetimeVisits
2008001 1 5 0
2008002 2 1 0
2008002 1 3 0
2008003 1 10 0

I want the end result to look like this:
DayNum TopicId Visits LifetimeVisits
2008001 1 5 5
2008002 2 1 1
2008002 1 3 8
2008003 1 10 18

Here is the query I€™ve been trying:

WITH ordered AS
(
SELECT *, ROW_NUMBER() OVER(PARTITION BY TopicId ORDER BY DayNum) AS RowNum
FROM TopicActivity
)
update ordered set
ordered.LifetimeVisits = ordered.Visits +
isnull((select o2.LifetimeVisits
from ordered as o2
where o2.TopicId = ordered.TopicId
and o2.RowNum = (ordered.RowNum - 1)),0)


The update doesn€™t seem to be happening in order as LifetimeVisits seems to have random results every time I run the query. (Usually the first couple of rows are right. I€™m dealing with thousands of records) I can€™t use a cursor as the query takes way to long.

I'm running SQL Server 2005.