Get max value within PARTITON BY in CTE function - sql

Question

I'm struggling with log aggregation with window functions. I have table with columns
Item (string)
User (string)
TimeStart (timestamp)
With a CTE, I'm trying to merge all entries within 5 min into one, to create a time interval. In C1 I'm looking for the start of next interval, C2 is grouping records into intervals. To this point it works fine.
When I selected C2 I got my data labeled with group ID. But when I'm trying to catch max time from my group as TimeEnd in C3 I got messed data that is useless. As I understand OVER PARTITION BY it should be a simple comparison of TimeStart values within partition and return of MAX, but instead of I get messed group IDs, and TimeEnds
Code below, SQL Server Express:
WITH C1 AS
(
SELECT
Item,
User,
TimeStart,
CASE
WHEN DATEDIFF(MINUTE, LAG(TimeStart) OVER (PARTITION BY Item, User ORDER BY TimeStart), TimeStart) < 5
THEN 0
ELSE 1
END AS isstart
FROM
Log
),
C2 AS
(
SELECT
*,
SUM(isstart) OVER (ORDER BY TimeStart ROWS UNBOUNDED PRECEDING) AS grp
FROM
C1
),
C3 AS
(
SELECT
*,
MAX(TimeStart) OVER (PARTITION BY grp) AS TimeEnd
FROM
C2
)
SELECT *
FROM C3
Could you explain to me what happened here and how to solve it?
PS: I could use GROUP BY clause but I'll lose non aggregated columns.

Answer 1

Thanks to #Alex, I got this solution:
WITH C1 AS
(
SELECT
Item,
User,
TimeStart,
CASE
WHEN DATEDIFF(MINUTE, LAG(TimeStart) OVER (PARTITION BY Item, User ORDER BY TimeStart), TimeStart) < 5
THEN 0
ELSE 1
END AS isstart
FROM
Log
),
C2 AS
(
SELECT
*,
SUM(isstart) OVER ( ORDER BY ItemPath,UserName,TimeStart
ROWS UNBOUNDED PRECEDING) as grp
FROM
C1
),
C3 AS(
SELECT *, MAX(TimeStart) OVER (PARTITION BY grp ) AS [TimeEnd] FROM C2
)
SELECT * FROM C3
where isstart=1
ORDER BY grp
The problem was in second CTE with the wrong ORDER of rows.
Also I send preview of output data
Output data preview