Merge consecutive time record in SQL server 2008 - sql

Question

Say I have data like this, timeslots is basically time with 30 mins apart.
Note there is a gap in 2021-12-24 between 15:30 and 16:30.
calender_date
timeslot
timeslot_end
2021-12-24
14:00:00
14:30:00
2021-12-24
14:30:00
15:00:00
2021-12-24
15:00:00
15:30:00
2021-12-24
16:30:00
17:00:00
2021-12-24
17:00:00
17:30:00
2021-12-24
17:30:00
18:00:00
2021-12-30
09:00:00
09:30:00
2021-12-30
09:30:00
10:00:00
I want to merge rows where timeslot_end = next row's timeslot and in the same day, so data would look like this.
calender_date
timeslot
timeslot_end
2021-12-24
14:00:00
15:30:00
2021-12-24
16:30:00
18:00:00
2021-12-30
9:00:00
10:00:00
I have try row numbering with self join,
WITH cte AS
(
SELECT
calender_date
,timeslot
,timeslot_end
,ROW_NUMBER()OVER(ORDER BY [timeslot])rn
FROM #tmp_leave tl
)
SELECT
MIN(a.timeslot) OVER(PARTITION BY a.calender_date, DATEDIFF(minute,a.timeslot_end,
ISNULL(b.timeslot, a.timeslot_end))) AS 'StartTime',
MAX(a.timeslot_end ) OVER(PARTITION BY a.calender_date,
DATEDIFF(minute,a.timeslot_end, ISNULL(b.timeslot, a.timeslot_end))) AS 'EndTime'
FROM cte a
LEFT JOIN cte b
ON a.rn + 1 = b.rn AND a.timeslot_end = b.timeslot
ORDER BY calender_date
But the result isn't quite right, it ignored the gap in 2021-12-24 and return below.
calender_date
timeslot
timeslot_end
2021-12-24
14:00:00
18:00:00
2021-12-30
9:00:00
10:00:00
I have been searching and trying to solve it for a while now, please help, any help is very very appreciated!!

Answer 1

This is a gaps and islands problem. We can approach this by creating pseudo groups for each island of continuous dates/times.
WITH cte AS (
SELECT *, LAG(timeslot_end) OVER
(ORDER BY calendar_date, timeslot) timeslot_end_lag
FROM yourTable
),
cte2 AS (
SELECT *, COUNT(CASE WHEN timeslot_end_lag <> timeslot THEN 1 END)
OVER (ORDER BY calendar_date, timeslot) AS grp
FROM cte
)
SELECT calendar_date,
MIN(timeslot) AS timeslot,
MAX(timeslot_end) AS timeslot_end
FROM cte2
GROUP BY calendar_date, grp
ORDER BY calendar_date;
Demo