Club overlapping time slots together in SQL Server - sql

Question

I have a table which has records of overlapping time-slots.
E.g.: Calendar(Id int, StartDate datetime, EndDate datetime, EventTitle nvarchar(100), ...)
The records would be like
01/20/2011 08:15 AM --- 01/20/2011 08:40 AM
01/20/2011 08:20 AM --- 01/20/2011 08:55 AM
01/20/2011 12:30 PM --- 01/20/2011 01:15 PM
01/20/2011 02:00 PM --- 01/20/2011 02:45 PM
01/20/2011 02:15 PM --- 01/20/2011 02:30 PM
01/21/2011 12:30 PM --- 01/21/2011 01:15 PM
.......
.......
The table represents the event schedule of a person
I want to club the overlapping slots together to give:
01/20/2011 08:15 AM --- 01/20/2011 08:55 AM
01/20/2011 12:30 PM --- 01/20/2011 01:15 PM
01/20/2011 02:00 PM --- 01/20/2011 02:45 PM
01/21/2011 12:30 PM --- 01/21/2011 01:15 PM
...
...
I am stuck at this part of the problem... (I removed my code that I pasted here, as it looks mangled. I am using stackoverflow for the first time.)
Part of my actual requirement is to find a time slot [t] (let's say 25 minutes) between two given dates, which I can accommodate into the schedule. [t] has to be found between 8:00 AM and 05:00 PM on any day.

Answer 1

This is an interesting question.
For a given date, lets consider the overlaps (I entered all my date times as per yours but for the 1/1/2011):
SELECT dt1.StartTime AS StartOverlap,
CASE
WHEN dt1.EndTime > dt2.EndTime THEN dt1.EndTime
ELSE dt2.EndTime
END AS EndOverlap
FROM datetimetest dt1, datetimetest dt2
WHERE dt2.StartTime > dt1.StartTime
AND dt2.StartTime < dt1.EndTime
This gives me:
StartOverlap EndOverlap
01/01/2011 08:15:00 01/01/2011 08:55:00
01/01/2011 14:00:00 01/01/2011 14:45:00
Great, we now know the start and end times we can't use that overlap.
How do we get the other none overlapping times?
I would look at all the time periods that are in the overlapping ranges, and then select out the id's that are not in that date range:
SELECT id
FROM (SELECT dt1.StartTime AS StartOverlap,
CASE
WHEN dt1.EndTime > dt2.EndTime THEN dt1.EndTime
ELSE dt2.EndTime
END AS EndOverlap
FROM datetimetest dt1, datetimetest dt2
WHERE dt2.StartTime > dt1.StartTime
AND dt2.StartTime < dt1.EndTime
) AS Overlaps, datetimetest dtt
WHERE dtt.StartTime >= Overlaps.StartOverlap
AND dtt.EndTime <= Overlaps.EndOverlap
This gives me a list of all the ids that are in the overlaps. I then just select all of the entries that are NOT in the overlaps:
SELECT StartTime, EndTime FROM datetimetest
WHERE id NOT IN(
SELECT id
FROM (SELECT dt1.StartTime AS StartOverlap,
CASE
WHEN dt1.EndTime > dt2.EndTime THEN dt1.EndTime
ELSE dt2.EndTime
END AS EndOverlap
FROM datetimetest dt1, datetimetest dt2
WHERE dt2.StartTime > dt1.StartTime
AND dt2.StartTime < dt1.EndTime
) AS Overlaps, datetimetest dtt
WHERE dtt.StartTime >= Overlaps.StartOverlap
AND dtt.EndTime <= Overlaps.EndOverlap)
To give me:
StartTime EndTime
01/01/2011 12:30:00 01/01/2011 13:15:00
I can then union both queries together to give me a list of all used time slots.
As we are effectively cross joining the same table, I would add an additional clause to each query to constrain the date to a single date.