Determine time gaps in SQL - sql

Question

I'm trying to self learn the lag and lead functions and thought I would try it with the following report but I'm not having much luck. My goal is to take some on-call start and stop times for a department and create a report detailing the time gaps for the day that there was no coverage. The department is assumed to have 24 hour coverage, 7 days a week. Is the only way to handle this to join it to a datetime table that has the date and every minute available? Any suggestions would be greatly appreciated.
The expected outcome for the data below would be:
on 09/01/2020 dept 3042300031 had a time gap from 20:59 to 21:00 and a time gap from 22:59 to 23:59
on 09/02/2020 dept 3042300031 had a time gap from 00:00 to 00:05 and a time gap from 20:59 to 22:00 and a time gap from 22:50 to 23:59
on 09/03/2020 dept 3042300031 had a time gap from 00:00 to 23:59
on 09/04/2020 dept 3042300031 had a time gap from 20:59 to 23:59
IF OBJECT_ID('tempdb..#report') IS NOT NULL DROP TABLE #report
GO
CREATE TABLE #report (
Contact_Date date
,Line int
,Start_Instant_dttm smalldatetime
,End_Instant_dttm datetime
,Asgn_to_Role int
,Asgn_to_Team bigint
);
INSERT INTO #report
SELECT
'9/1/2020',1,'9/1/2020 00:00','9/1/2020 20:59',270,3042300031
UNION
SELECT
'9/1/2020',2,'9/1/2020 21:00','9/1/2020 22:59',270,3042300031
UNION
SELECT
'9/2/2020',1,'9/2/2020 00:05','9/2/2020 20:59',270,3042300031
UNION
SELECT
'9/2/2020',2,'9/2/2020 22:00','9/2/2020 22:59',270,3042300031
UNION
SELECT
'9/4/2020',1,'9/4/2020 00:00','9/4/2020 20:59',270,3042300031;

Answer 1

Finding the gaps is simple:
with cte as
(
select Asgn_to_Team, Start_Instant_dttm, End_Instant_dttm
,lag(End_Instant_dttm)
over (partition by Asgn_to_Team
order by Start_Instant_dttm) as prev_end
from #report
)
select Asgn_to_Team, prev_end as gap_start, Start_Instant_dttm as gap_end
from cte
where prev_end < Start_Instant_dttm
But splitting them into days is much harder, you need to join to a calendar table:
with cte as
(
select Asgn_to_Team, Start_Instant_dttm, End_Instant_dttm
,lag(End_Instant_dttm)
over (partition by Asgn_to_Team
order by Start_Instant_dttm) as prev_end
from #report
)
select Asgn_to_Team,
case when prev_end > cast(cal.cal_date as datetime)
then prev_end
else cast(cal.cal_date as datetime)
end as gap_start,
case when Start_Instant_dttm < cast(dateadd(day, 1, cal.cal_date) as datetime)
then Start_Instant_dttm
else cast(dateadd(day, 1, cal.cal_date) as datetime)
end as gap_end
from cte join cal -- one row for each date covered
on cast(cal.cal_date as datetime) <= Start_Instant_dttm
and cast(dateadd(day, 1, cal.cal_date) as datetime) > prev_end
where prev_end < Start_Instant_dttm
Hopefully I got the >/< right, see fiddle