How to make entries from column appear as row title - sql

Question

I have a hospital database which looks something like this
id | patient_name | admitDate | DischargeDate |RoomCategory
1 | john |3/01/2011 | 5/01/2011 |Category1
2 | lisa |3/01/2011 | 4/01/2011 |Category2
3 | ron |5/01/2011 | 10/01/2011 |Category1
4 | howard |6/01/2012 | 10/01/2012 |Category3
5 | john |6/05/2011 | 7/05/2011 |Category4
6 | rammy |6/02/2011 | 7/03/2011 |Category4
I have to calculate the number of patients in hospital on each day (both admit and discharge date to be counted) and group them by category
Suppose on 3/01/2011 we have 2 patients, one in category 1 and one in category 2 on 4/01/2011 we again have same 2 patients but on 5/01/2011 lisa (id 2) is discharged so we only have 1 patient from category 1 but now ron (id 3) is also admitted so now we also have to count him.
The output should look something like this
Date | Category1 | Category2 | Category3 |Category4
3/01/2011 | 1 | 1 | 0 | 0
4/01/2011 | 1 | 1 | 0 | 0
5/01/2011 | 2 | 0 | 0 | 0
I am not able to figure out how to list all the dates which might have a patient, because the actual table is huge and a lot of dates don't have any patient. I also am not able to get how will I count distinctively to get count under each category.
I have 15 categories in total in my actual table so using where for each one of them separately wouldn't be very efficient.

Answer 1

You have 2 problems here. 1 you need a calendar table, and then 2 a pivot. I suggest, if I am honest, you invest in creating a calendar table firstly, but I use an inline one here. Then you can use pivoting to convert the values to columns. I use conditional aggregation here, as it is transferable and less restrictive.
SELECT *
INTO dbo.YourTable
FROM (VALUES(1,'john ',CONVERT(date,'3/01/2011'),CONVERT(date,'5/01/2011 '),'Category1'),
(2,'lisa ',CONVERT(date,'3/01/2011'),CONVERT(date,'4/01/2011 '),'Category2'),
(3,'ron ',CONVERT(date,'5/01/2011'),CONVERT(date,'10/01/2011'),'Category1'),
(4,'howard',CONVERT(date,'6/01/2012'),CONVERT(date,'10/01/2012'),'Category3'),
(5,'john ',CONVERT(date,'6/05/2011'),CONVERT(date,'7/05/2011 '),'Category4'),
(6,'rammy ',CONVERT(date,'6/02/2011'),CONVERT(date,'7/03/2011 '),'Category4'))V(id,patient_name,admitDate,DischargeDate,RoomCategory)
GO
WITH N AS(
SELECT N
FROM (VALUES(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL))N(N)),
Tally AS(
SELECT 0 AS I
UNION ALL
SELECT TOP (SELECT DATEDIFF(DAY, MIN(admitDate), MAX(DischargeDate)) FROM dbo.YourTable)
ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) AS I
FROM N N1, N N2, N N3), --UP to 1000 days. Add more cross joins for more days
Calendar AS(
SELECT DATEADD(DAY, T.I, YT.MinAdmitDate) AS D
FROM Tally T
CROSS APPLY (SELECT MIN(admitDate) AS MinAdmitDate FROM dbo.YourTable) YT)
SELECT C.D AS [Date],
COUNT(CASE YT.RoomCategory WHEN 'Category1' THEN 1 END) AS Category1,
COUNT(CASE YT.RoomCategory WHEN 'Category2' THEN 1 END) AS Category2,
COUNT(CASE YT.RoomCategory WHEN 'Category3' THEN 1 END) AS Category3,
COUNT(CASE YT.RoomCategory WHEN 'Category4' THEN 1 END) AS Category4
FROM Calendar C
LEFT JOIN dbo.YourTable YT ON C.D >= YT.admitDate
AND C.D <= DischargeDate
GROUP BY C.D;
GO
DROP TABLE dbo.YourTable;
db<>fiddle Note that that results might not be what you expect as DB Fiddle defaults to American, and you provide an ambiguous date format and I don't provide an explicit style in the CONVERT functions.