Columns columns by time in a sql table - sql

Question

I have a sql table as follows:
+-----------+----------+----------+---------------+
| AccountID | PersonId | DoctorID | Admitdatetime |
+-----------+----------+----------+---------------+
| 1 | 2 | 345 | 20090108 |
| 2 | 3 | 53 | 20090109 |
| 3 | 1 | 234 | 20090110 |
| 4 | 2 | 345 | |
+-----------+----------+----------+---------------+
Each row of this table is like a visit of a patient given by the admitdatetime. Each unique record is referenced by AccountID
Date column is basically int and is yyyymmdd. So just subtracting two dates might not be right as it is not datetime. I just checked.
Now, what I want to do for each record in the table is to add 3 columns. One for last three months, one for last 6 months, and one for last 12 months.
The columns are described as follows:
The no. of cases a DoctorID has seen in the past 3 months of that current record. Similarly, no. of cases a DoctorID has seen in the past 6 months of that current record.
I am doing a self join like this:
SELECT a.DoctorID, count(AccountID) FROM
Visits AS a INNER JOIN
Visits AS b ON a.DoctorId = b.DoctorId
WHERE a.admitdatetime - b.admitdatetime <= 90
The above one I am doing for the 3 months case, but I don't think it is right. I want for each record the no. of cases (count of AccountId) a doctor has seen 3,6,9 months before that. So for each DoctorID, that value would vary based on which record the doctorID is present and it's 3,6,9 months prior that admitdatetime of that record such that the above code would just give me one value for a doctorID. That doesn't seem right.
I think the join should be grouped by DoctorId, AccountId as I need to join all the doctorid back to each record and each record is identified by accountid. So then join it back on doctorid and accountid. Does this sound right?

Answer 1

I would suggest correlated subqueries:
select v.*,
(select sum(case when v2.AdmitDate >= v.AdmitDate - interval '3 month'
then 1 else 0
end)
from visits v2
where v2.doctorid = v.doctorid
) as last3,
(select sum(case when v2.AdmitDate >= v.AdmitDate - interval '6 month'
then 1 else 0
end)
from visits v2
where v2.doctorid = v.doctorid
) as last6,
(select sum(case when v2.AdmitDate >= v.AdmitDate - interval '12 month'
then 1 else 0
end)
from visits v2
where v2.doctorid = v.doctorid
) as last12
from visits v;
I should point out that Postgres allows you to simplify this syntax:
(select sum((v2.AdmitDate >= v.AdmitDate - interval '3 month')::int)
from visits v2
where v2.doctorid = v.doctorid
) as last3,
And in more recent versions of Postgres you can use a lateral join to combine the logic into a single subquery.
EDIT:
A reasonable simplification of the query is:
select v.*,
(select count(*)
from visits v2
where v2.doctorid = v.doctorid and v2.AdmitDate >= v.AdmitDate - interval '3 month'
) as last3,
(select count(*)
from visits v2
where v2.doctorid = v.doctorid and 2.AdmitDate >= v.AdmitDate - interval '6 month'
) as last6,
(select count(*)
from visits v2
where v2.doctorid = v.doctorid and v2.AdmitDate >= v.AdmitDate - interval '12 month'
) as last12
from visits v;