Select overlapping holidays per department - sql

Question

I'm trying to check if any of the employees at my company are requesting overlapping holidays. It's the policy that only 1 person per department is allowed to be off at once.
What query should I use? (I am a noob, so tell me if you need more information).
An Example of the table I want to use:
Request ID Employee ID Department ID Start Date End Date
1 10 1 2015-12-20 2016-12-27
2 10 1 2016-06-01 2015-06-14
3 11 1 2015-12-26 2015-12-27
4 11 1 2016-06-09 2016-06-23
5 12 2 2015-12-26 2015-12-26
6 12 2 2016-07-01 2016-07-14
Results:
Request ID Status
1 Not Approved, overlapping 26-27/12
2 Not Approved, overlapping 09-14/06
3 Not Approved, overlapping completely
4 Not Approved, overlapping 09-14/06
5 Approved, not overlapping in this department
6 Approved, not overlapping in this department
In the second phase, I want to compare if the holidays requested, are within a week, containing a bank holiday (I will have a different table with the bank holidays).
Thanks in advance!

Answer 1

One way is with exists:
select e.*,
(case when exists (select 1
from example e2
where e2.departmentid = e.departmentid and
e2.employeeid <> e.employeeid and
e2.startdate <= e.enddate and
e2.enddate >= e.startdate
)
then 'Overlapping'
else 'NotOverlapping'
end) as Status
from example e;
Getting your full message is trickier and depends on the database. The problem are multiple overlaps.
Actually, we can get more information without too much problem:
select e.RequestId,
(case when count(e2.RequestId) = 0
then 'Approved, not overlapping in this department'
when count(e2.RequestId) = 1
then (case when min(e2.startdate) <= e.startdate and
max(e2.enddate) >= e.enddate
then 'Not Approved, overlapping completely'
else 'Not Approved, overlapping partially'
end)
else 'Not Approved, multiple overlaps'
end) as Status
from example e left join
example e2
on e2.departmentid = e.departmentid and
e2.startdate <= e.enddate and
e2.enddate >= e.startdate and
e2.employeeid <> e.employeeid
group by e.RequestId, e.startdate, e.enddate;
Getting the actual dates is trickier, without knowing the database.

Answer 2

create table #holidays(RequestID int, EmployeeID int, DepartmentID int, StartDate date, EndDate date)
insert into #holidays
select 1, 10, 1, '2015-12-20', '2016-12-27'
union select 2, 10, 1, '2016-06-01', '2015-06-14'
union select 3, 11, 1, '2015-12-26', '2015-12-27'
union select 4, 11, 1, '2016-06-09', '2016-06-23'
union select 5, 12, 2, '2015-12-26', '2015-12-26'
union select 6, 12, 2, '2016-07-01', '2016-07-14'
with CTE
as (
select T1.RequestID,max(T2.RequestID) as T2RequestID, count(*) as cnt
from #holidays T1
join #holidays T2 on
T1.DepartmentID = T2.DepartmentID
and T1.StartDate < T2.EndDate and T1.EndDate > T2.StartDate
and T1.RequestID <> T2.RequestID
group by T1.RequestID
)
select H.*,
case
when isnull(CTE.cnt,0) = 0 then 'Approved, not overlapping in this department'
else 'Not Approved, overlapping ' + convert(nvarchar(50),H2.StartDate) + ' - ' + convert(nvarchar(50),H2.EndDate)
end as Res
from #holidays H
left join CTE on H.RequestID = CTE.RequestID
left join #holidays H2 on H2.RequestID = CTE.T2RequestID
Btw, first employer have too big holidays - more than 1 year long :)
Another StartDate > EndDate (what is it ?)
Result