POSTGRESQL Comparing a time overlap across rows - postgresql

Question

I'm creating a query in our EHR. I am looking for patients that have accounts that are within 48 hours of each other to make sure they aren't coming back in with the same symptoms.
to work with i have the following columns in two tables
visit table
'patnum' which is the unique number attached to each visit
'mr_num' which is the unique number attached to each patient
.
er_log table
'patnum' which is the unique number attached to each visit
arrivaldt and arrivaltime which i will unite in a timestamp and is when the patient presented themselves
dischargedate and dischargetime which I will unite in a timestamp and is when the patient was discharged
1.what are some methods for comparing the arrivaltime and dischargetime across all of an mr_num's accounts and finding those with less than 48 hours of difference?
thanks!
edit: my explanation may be a little vague
example pt with MR num '334455' is a frequent flyer at the ER. she has come in 25 times in the last 7 months. each of those visits has a unique patnum, an arrivaldate and a discharge date. I am trying to find if any of her visits happened within 48 hours of each other by comparing one visits date with every other visits arrival date. query will report those visits that are within 48hours of each other.

Answer 1

I'm assuming that dischargetime and arrivaltime are of type timestamp to simplify the code, but you can always do the conversion on your own.
Previous answer
Use INNER JOIN to connect with table containing your times. Assuming that a patient can be discharged only after he presented himself, you can subtract arrivaltime from dischargetime to receive result in INTERVAL type and get back only those rows where they differ by maximum 2 days, which is 48 hours.
SELECT
v.mr_num, v.patnum
FROM
visit v
INNER JOIN er_log e USING ( patnum )
WHERE
v.mr_num = ?
AND e.dischargetime - e.arrivaltime < interval '2 days'
To get some deeper understanding you could look into SQL Fiddle I've created.
Current answer
Changes in code after comment and edited question. This will scan all visits for a particular patient and return those that happened within 2 days from the end of any of previous visits. You may want to change logic a bit with the line where the comparison takes place, to better suit your need
AND e1.dischargetime - e2.arrivaltime < interval '2 days'
and decide whether you base your times on arrivaltime or dischargetime for the current visit being compared and all the visits that are in a lookup.
SELECT
v.mr_num, v.patnum
FROM
visit v1
INNER JOIN er_log e1 USING ( patnum )
WHERE
v.mr_num = ?
AND EXISTS (
SELECT 1
FROM visit v2
INNER JOIN er_log e2 USING ( patnum )
WHERE
v1.mr_num = v2.mr_num
AND v1.patnum IS DISTINCT FROM v2.patnum
AND e1.dischargetime - e2.arrivaltime < interval '2 days'
)

Answer 2

What you will need to do is do TWO inner joins to the ER_LOG table. I'm going to borrow the previous answer and extend it:
SELECT
v1.mr_num, v1.patnum, v2.patnum,
(e1.dischargetime - e2.arrivaltime) as elapsed
FROM
visit v1
INNER JOIN er_log e1 on (v1.patnum = e1.patnum)
INNER JOIN visit v2 on (v1.mr_num = v2.mr_num and v1.patnum <> v2.patnum)
INNER JOIN er_log e2 on (v2.patnum = e2.patnum)
WHERE
v1.mr_num = ?
AND e1.dischargetime - e2.arrivaltime < interval '2 days'
You must join against the er_log table twice to get the two different visits so that you can compare times. You must also join against visit again to establish visits for the same patient (mr_num), and you may exclude the identical visit by making sure the patnum values are different.