How to identify MIN value for records within a rolling date range in SQL - sql

Question

I am trying to calculate a MIN date by Patient_ID for each record in my dataset that dynamically references the last 30 days from the date (Discharge_Dt) on that row. My initial thought was to use a window function, but I opted for a subquery, which is close, but not quite what I need.
Please note, my sample query is also missing logic that limits the MIN Discharge_Dt to the last 30 days, in other words, I do not want a MIN Discharge_Dt that is older than 30 days for any given row.
Sample Query:
SELECT Patient_ID,
Discharge_Dt,
/* Calculating the MIN Discharge_Dt by Patient_ID for the last 30
days based upon the Discharge_Dt for that row */
(SELECT MIN(Discharge_Dt)
FROM admissions_ds AS b
WHERE a.Patient_ID = b.Patient_ID AND
a.Discharge_Dt >= DATEADD('D', -30, GETDATE())) AS MIN_Dt
FROM admissions_ds AS a
Desired Output Table:
Patient_ID | Discharge_Dt | MIN_Dt
10 | 2017-08-15 | 2017-08-15
10 | 2017-08-31 | 2017-08-15
10 | 2017-09-21 | 2017-08-31
15 | 2017-07-01 | 2017-07-01
15 | 2017-07-18 | 2017-07-01
20 | 2017-05-05 | 2017-05-05
25 | 2017-09-24 | 2017-09-24

Answer 1

Here you go,
Just a simple join required.
drop TABLE if EXISTS admissions_ds;
create table admissions_ds (Patient_ID int,Discharge_Dt date);
insert into admissions_ds
values
(10,'2017-08-15'),
(10,'2017-08-31'),
(10,'2017-09-21'),
(15,'2017-07-01'),
(15,'2017-07-18'),
(20,'2017-05-05'),
(25,'2017-09-24');
select t1.Patient_ID,t1.Discharge_Dt,min(t2.Discharge_Dt) as min_dt
from admissions_ds as t1
join admissions_ds as t2 on t1.Patient_ID=t2.Patient_ID and t2.Discharge_Dt > t1.Discharge_Dt - interval '30 days'
group by 1,2
order by 1,2
;