SQL window functions to find the last row of a joined table - sql

Question

I have tables as follows in a MariaDB 10.3 server.
person (id, name)
donation (id, person_id, amount, date_given, campaign)
With a one to many relationship from person to donation. Imagine 'campaign' describes what the donation was for, but let's just use campaign names A and B. There are only these 2.
I want a query that returns a row for each person, with columns being all the columns from their most recent donation to A and their most recent donation to B.
e.g. (this code is wrong, but may communicate better than my words!)
WITH lastDonationA AS (
SELECT *
FROM donation
WHERE campaign = 'A' AND /* IS LAST ROW */
GROUP BY person_id
)
WITH lastDonationB AS (
SELECT *
FROM donation
WHERE campaign = 'B' AND /* IS LAST ROW */
GROUP BY person_id
)
SELECT person.name, lastDonationA.*, lastDonationB.*
FROM person
LEFT JOIN lastDonationA ON lastDonationA.person_id = person.id
LEFT JOIN lastDonationB ON lastDonationB.person_id = person.id
;
I have a hunch that a window function is going to be good for this, but I can't quite fathom it!

Answer 1

I think I was confused by the FIRST_VALUE window function that sounds perfect but isn't for this use case.
WITH orderedDonationsForA AS (
SELECT *, ROW_NUMBER() OVER (PARTITION BY person_id ORDER BY date_given DESC) rn
FROM donation
WHERE campaign = 'A'
),
orderedDonationsForB AS (
SELECT *, ROW_NUMBER() OVER (PARTITION BY person_id ORDER BY date_given DESC) rn
FROM donation
WHERE campaign = 'B'
)
SELECT person.*, lastAdonation.*, lastBdonation.*
FROM person
LEFT JOIN orderedDonationsForA lastAdonation
ON lastAdonation.person_id AND lastAdonation.rn = 1
LEFT JOIN orderedDonationsForB lastBdonation
ON lastAdonation.person_id AND lastBdonation.rn = 1
This works by numbering the rows ordered within each person from most recent to oldest, then the join picks off the first of these by the ON clause.