The query should have a parameter with a start date and an end date. Arrival Date and Departure date are DateTime fields. Between the start date and the end date, the query should show the number of PATIENT_IDs per hour. So if the record of the PATIENT_ID had an ARRIVAL_DATE of 01/16/2018 04:23:00 am and a DEPARTURE_DATE of 01/18/2018 12:31:00 pm, then in any given hour between 01/16/2018 04:23:00 am and 01/18/2018 12:31:00 pm this PATIENT_ID would count a 1. From the start date to the end date I need,per hour, to find the average number of PATIENT_IDs PRESENT.
OK, so based on the advice you've received so far, what have you tried?
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.