Select rows for last n days after event occurs - sql

Question

I have the following table and data:
PatientID PatientName Diagnosed ReportDate ...
1 0
1 0
1 0
1 1
So there are multiple rows for each patient, as the reports come few times a day.
Whenever the diagnosed field is changed to 1, for that patient, I'd like to get the past 3 days of data . So when Diagnosed ==1, get report time -3 days of data for each patient.
SELECT Patients.ReportDate
FROM Patients
WHERE Diagnosed = 1 and date > ReportDate - interval '3' day;
So getting the past 3 days of data, can be done with ReportDate - interval time, but how do I specify that for every patient (since multiple ids can be for that patient) based on the diagnosed field?
I usually do this filtering after getting csvs in python, but the data set is too large, so I'd like to filter before I convert them to dataframes.

Answer 1

You can look at this another way, which is whether diagnosed = 1 in the next three days -- and take all rows where that is true:
select p.*
from (select p.*,
count(*) filter (where diagnosed = 1) over (partition by patientId order by reportDate range between interval '0 day' following and interval '3 day' following) as cnt_diagnosed_3
from patients p
) p
where cnt_diagnosed_3 > 0
order by patientId, reportDate;

Answer 2

Whenever the diagnosed field is changed to 1, for that patient, I'd like to get the past 3 days of data.
SELECT (p).*
FROM (
SELECT p
, diagnosed
, bool_or(diagnosed = 1) OVER (w RANGE BETWEEN CURRENT ROW AND '3 days' FOLLOWING) AS in_range
, lag(diagnosed) OVER w AS last_diagnosed
FROM patients p
WINDOW w AS (PARTITION BY patientid ORDER BY reportdate)
) sub
WHERE diagnosed = 0 AND in_range
OR diagnosed = 1 AND last_diagnosed = 0
ORDER BY patientid, reportdate;
db<>fiddle here
Returns the "past 3 days of data" where the "field is changed to 1" (previous row had "0").
The WINDOW clause is just syntactic sugar to avoid spelling out the same window definition repeatedly. (No additional benefit for performance.)
SELECT p in the innermost subquery is a neat way to get the whole row. The outer SELECT (p).* returns complete rows without auxiliary columns added in the subquery. This way we get whole rows without spelling out all columns (or even needing to know all of them).
RANGE distance PRECEDING/FOLLOWING requires Postgres 11 or later.
Here is a slower alternative that also works for older versions:
SELECT p.*
FROM (
SELECT patientid, reportdate
FROM (
SELECT patientid, reportdate, diagnosed
, lag(diagnosed) OVER (PARTITION BY patientid ORDER BY reportdate) AS last_diagnosed
FROM patients
) p0
WHERE diagnosed = 1
AND last_diagnosed = 0
) d
JOIN patients p USING (patientid)
WHERE p.reportdate BETWEEN d.reportdate - interval '3 days' AND d.reportdate
ORDER BY p.patientid, p.reportdate;
Subquery d select rows where Diagnosed just switched to 1. Then self-join to select your time frame.
For gaps-and-islands basics, see:
Select longest continuous sequence
You also added:
So when Diagnosed ==1, get report time -3 days of data for each patient.
That's a wider definition, and that's what Gordon's query does. Goes to show the importance of an exact definition of requirements.