SQL Server 2017

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create table patients (patientid int, admitted date, discharged date);
insert into patients values 
(1, '2019-01-12', '2019-02-03'), 
(2, '2019-02-13', null),
(3, '2019-01-12', '2019-01-31'), 
(4, '2019-01-14', null); 
4 rows affected

SELECT DATEADD(mm, DATEDIFF(mm, 0, GETDATE()), 0)


(No column name)


2023-01-01 00:00:00.000


select 
  p.patientid, 
  case 
    when discharged < dateadd(mm, datediff(mm, 0, getdate()), 0) then -1
    else datediff(day, 
      case month(admitted) 
        when month(getdate()) then admitted
        else dateadd(mm, datediff(mm, 0, getdate()), 0)
      end,
      case 
        when discharged is null or discharged > getdate() then getdate()
        else discharged
      end
     )  
  end + 1 daysincurmonth  
from patients p


patientid
daysincurmonth


1
0

2
9

3
0

4
1457