Oracle SQL Paired Data - sql

Question

I need to find the difference in averages between patient weights at different visits (time points), but I'm struggling with finding the "paired" averages:
I have 1 table (PHYS) containing patient weights at different visits:
PATIENT VISIT WEIGHT
1 Baseline 200
1 1 Month 190
1 2 Month 170
2 Baseline 300
2 1 Month 290
2 2 Month 280
3 Baseline 250
3 1 Month 230
My problem is that I only want to find the difference for paired data. For example, when calculating the amount of weight loss between the 2 month and Baseline visits, I would want to find the difference between the (average 2 Month weight) and the (average Baseline weight FOR ONLY THOSE PATIENTS WITH A 2 MONTH WEIGHT). In this example, the result should be AVG(170,280) - AVG(200,300) = -25 (since only patient 1 and 2 have 2 Month weights).
Here is what I have, but it calculates the difference based on all weights:
SELECT VISIT
AVG(WEIGHT)
-
(SELECT
AVG(WEIGHT)
FROM PHYS
WHERE VISIT = 'BASELINE')
FROM PHYS
GROUP BY VISIT
My desired output would be (I know I need to add an ORDER BY):
VISIT CHANGE FROM BASELINE
Baseline 0
1 Month -13.3
2 Month -25
Thank you and sorry for such a newb question.

Answer 1

You can do this with a join to the same table but only for the 'Baseline'. Then, the aggregation only aggregates the values that match, so you should get different baseline averages for the three groups (because the populations are different):
select p.visit, avg(p.weight) as avg_weight, avg(pbl.weight) as avg_blweight,
(avg(p.weight) - avg(pbl.weight)) as change
from phys p join
phys pbl
on p.patient = pbl.patient and
pbl.visit = 'Baseline'
group by p.visit;