NUmber of Patients in an A&E department every hour

Question

Hi SQL Guru's,
I've got what for me is a tricky little problem. I want to be able to count the number of patients in our A&E department at every hour of every day.
I have access to the A&E Attendances tables that list the PatientID, ArrivalDateTime, DepartureDateTime etc so what I want to do is for example, look at how many people were in the department at 13:00hrs yesterday. This will be any patient that has arrived
before 13:00hrs and as yet does not have a departure date time. I want this to loop and do this calulation for every hour and every day between what ever start and end date parameters I pass into the query
Any help on this would be greatly appreciated
Many Thanks Guys
AntBai

Answer 1

Please try this: 
declare @current_date datetime = '2014-11-04 04:10'
declare @last13hrs datetime = @current_date - '13:00' --select @last13hrs
declare @Attendances table
( PatientID int, ArrivalDateTime datetime, DepartureDateTime datetime);
insert @Attendances
values
( 1, '2014-11-03 10:00', '2014-11-04 01:00'),
( 2, '2014-11-03 11:00', null),
( 3, '2014-11-03 18:00', '2014-11-04 03:00'),
( 4, '2014-11-03 23:10', '2014-11-04 04:00'),
( 5, '2014-11-03 23:10', null),
( 6, '2014-11-03 23:20', Null);
--This will be any patient that has arrived before 13:00hrs and as yet does not have a departure date time.
select
count(*) over() as cnt
from
@Attendances
where
ArrivalDateTime >= @last13hrs
and DepartureDateTime is null;
--more data to show
select
datediff(hh, ArrivalDateTime, DepartureDateTime ),
datediff(hh, ArrivalDateTime, @current_date )
from @Attendances;
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