Collapse multiple rows based on time values - sql

Question

I'm trying to collapse rows with consecutive timeline within the same day into one row but having an issue because of gap in time. For example, my dataset looks like this.
Date StartTime EndTime ID
2017-12-1 09:00:00 11:00:00 12345
2017-12-1 11:00:00 13:00:00 12345
2018-09-08 09:00:00 10:00:00 78465
2018-09-08 10:00:00 12:00:00 78465
2018-09-08 15:00:00 16:00:00 78465
2018-09-08 16:00:00 18:00:00 78465
As up can see, the first two rows can just be combined together without any issue because there's no time gap within that day. However. for the entries on 2019-09-08, there is a gap between 12:00 and 15:00. And I'd like to merge these four records into two different rows like this:
Date StartTime EndTime ID
2017-12-1 09:00:00 13:00:00 12345
2018-09-08 09:00:00 12:00:00 78465
2018-09-08 15:00:00 18:00:00 78465
In other words, I only want to collapse the rows only when the time variables are consecutive within the same day for the same ID.
Could anyone please help me with this? I tried to generate unique group using LAG and LEAD functions but it didn't work.

Answer 1

You can use a recursive cte. Group it as same group if the EndTime is same as next StartTime. And then find the MIN() and MAX()
with cte as
(
select rn = row_number() over (partition by [ID], [Date] order by [StartTime]),
*
from tbl
),
rcte as
(
-- anchor member
select rn, [ID], [Date], [StartTime], [EndTime], grp = 1
from cte
where rn = 1
union all
-- recursive member
select c.rn, c.[ID], c.[Date], c.[StartTime], c.[EndTime],
grp = case when r.[EndTime] = c.[StartTime]
then r.grp
else r.grp + 1
end
from rcte r
inner join cte c on r.[ID] = c.[ID]
and r.[Date] = c.[Date]
and r.rn = c.rn - 1
)
select [ID], [Date],
min([StartTime]) as StartTime,
max([EndTime]) as EndTime
from rcte
group by [ID], [Date], grp
db<>fiddle demo

Answer 2

Unless you have a particular objection to collapsing non-consecutive rows, which are consecutive for that ID, you can just use GROUP BY:
SELECT
Date,
StartTime = MIN(StartTime),
EndTime = MAX(EndTime),
ID
FROM table
GROUP BY ID, Date
Otherwise you can use a solution based on ROW_NUMBER:
SELECT
Date,
StartTime,
EndTime,
ID
FROM (
SELECT *,
rn = ROW_NUMBER() OVER (PARTITION BY Date, ID ORDER BY StartTime)
FROM table
) t
WHERE rn = 1

Answer 3

This is an example of a gaps-and-islands problem -- actually a pretty simple example. The idea is to assign an "island" grouping to each row specifying that they should be combined because they overlap. Then aggregate.
How do you assign the island? In this case, look at the previous endtime and if it is different from the starttime, then the row starts a new island. Voila! A cumulative sum of the the start flag identifies each island.
As SQL:
select id, date, min(starttime), max(endtime)
from (select t.*,
sum(case when prev_endtime = starttime then 0 else 1 end) over (partition by id, date order by starttime) as grp
from (select t.*,
lag(endtime) over (partition by id, date order by starttime) as prev_endtime
from t
) t
) t
group by id, date, grp;
Here is a db<>fiddle.
Note: This assumes that the time periods never span multiple days. The code can be very easily modified to handle that . . . but with a caveat. The start and end times should be stored as datetime (or a related timestamp) rather than separating the date and times into different columns. Why? SQL Server doesn't support '24:00:00' as a valid time.