Performing INSERT for each row in a select RESULT - sql-server

Question

First, a general description of the problem: I'm running a periodical process which updates total figures in a table. The issue is, that multiple updates may be required in each execution of the process, and each execution depends on the previous results.
My question is, can it be done in a single SQL Server SP?
My code (I altered it a little to simply the sample):
INSERT INTO CustomerMinuteSessions(time, customer, sessions, bytes, previousTotalSessions)
SELECT MS.time,
MS.customer,
MS.totalSessions,
MS.totalBytes,
CTS.previousTotalSessions
FROM (SELECT time, customer, SUM(sessions) as totalSessions, SUM(bytes) AS totalBytes
FROM MinuteSessions
WHERE time > #time
GROUP BY time, x) MS
CROSS APPLY TVF_GetPreviousCustomerTotalSessions(MS.customer) CTS
ORDER BY time
The previousTotalSessions column depends on other rows in UpdatedTable, and its value is retrieved by CROSS APPLYing TVF_GetPreviousCustomerTotalSessions, but if I execute the SP as-is, all the rows use the value retrieved by the function without taking the rows added during the execution of the SP.
For the sake of completeness, here's TVF_GetPreviousCustomerTotalSessions:
FUNCTION [dbo].[TVF_GetCustomerCurrentSessions]
(
#customerId int
)
RETURNS #result TABLE (PreviousNumberOfSessions int)
AS
BEGIN
INSERT INTO #result
SELECT TOP 1 (PreviousNumberOfSessions + Opened - Closed) AS PreviousNumberOfSessions
FROM CustomerMinuteSessions
WHERE CustomerId = #customerId
ORDER BY time DESC
IF ##rowcount = 0
INSERT INTO #result(PreviousNumberOfSessions) VALUES(0)
RETURN
END
What is the best (i.e. without for loop, I guess...) to take previous rows within the query for subsequent rows?

Answer 1

If you are using SQL-2005 and later, you can do it with few CTEs in one shot. If you use SQL-2000 you'll can use inline table-valued function.
Personally I like the CTE approach more, so I'm including a schematic translation of your code to CTEs syntax. (Bare in mind hat I didn't prepare a test set to check it).
WITH LastSessionByCustomer AS
(
SELECT CustomerID, MAX(Time)
FROM CustomerMinuteSessions
GROUP BY CustomerID
)
, GetPreviousCustomerTotalSessions AS
(
SELECT LastSession.CustomerID, LastSession.PreviousNumberOfSessions + LastSession.Opened - LastSession.Closed AS PreviousNumberOfSessions
FROM CustomerMinuteSessions LastSession
INNER JOIN LastSessionByCustomer ON LastSessionByCustomer.CustomerID = LastSession.CustomerID
)
, MS AS
(
SELECT time, customer, SUM(sessions) as totalSessions, SUM(bytes) AS totalBytes
FROM MinuteSessions
WHERE time > #time
GROUP BY time, x
)
INSERT INTO CustomerMinuteSessions(time, customer, sessions, bytes, previousTotalSessions)
SELECT MS.time,
MS.customer,
MS.totalSessions,
MS.totalBytes,
ISNULL(GetPreviousCustomerTotalSessions.previousTotalSessions, 0)
FROM MS
RIGHT JOIN GetPreviousCustomerTotalSessions ON MS.Customer = GetPreviousCustomerTotalSessions.CustomerID
Going a bit beyond your question, I think that your query with cross apply could make big damage to the database once table CustomerMinuteSessions database grows
I would add an index like to improve your chances of getting Index-Seek:
CREATE INDEX IX_CustomerMinuteSessions_CustomerId
ON CustomerMinuteSessions (CustomerId, [time] DESC, PreviousNumberOfSessions, Opened, Closed );