Question

I have hospital patient admission data in Microsoft SQL Server r2 that looks something like this:

PatientID, AdmitDate,        DischargeDate
Jones.     1-jan-13 01:37.   1-jan-13 17:45
Smith      1-jan-13 02:12.   2-jan-13 02:14
Brooks.    4-jan-13 13:54.   5-jan-13 06:14

I would like count the number of patients in the hospital day by day and hour by hour (ie at

1-jan-13 00:00. 0
1-jan-13 01:00. 0
1-jan-13 02:00. 1
1-jan-13 03:00. 2

And I need to include the hours when there are no patients admitted in the result.

I can’t create tables so making a reference table listing all the hours and days is out, though.

Any suggestions?

Answer 1

To solve this problem, you need a list of date-hours. The following gets this from the admit date cross joined to a table with 24 hours. The table of 24 hours is calculating from information_schema.columns — a trick for getting small sequences of numbers in SQL Server.

The rest is just a join between this table and the hours. This version counts the patients at the hour, so someone admitted and discharged in the same hour, for instance is not counted. And in general someone is not counted until the next hour after they are admitted:

with dh as (
     select DATEADD(hour, seqnum - 1, thedatehour ) as DateHour
     from (select distinct cast(cast(AdmitDate as DATE) as datetime) as thedatehour
           from Admission a
          ) a cross join
          (select ROW_NUMBER() over (order by (select NULL)) as seqnum
           from INFORMATION_SCHEMA.COLUMNS
          ) hours
          where hours <= 24
    )
select dh.DateHour, COUNT(*) as NumPatients
from dh join
     Admissions a
     on dh.DateHour between a.AdmitDate and a.DischargeDate
group by dh.DateHour
order by 1

This also assumes that there are admissions on every day. That seems like a reasonable assumption. If not, a calendar table would be a big help.