Aggregate calculation query on single SQL table - mysql

Question

In my database I have a table, payers_payments, consisting of some foreign keys (payer_id, payment_id) and some pivot fields (amount, pays).
| id | payer_id | payment_id | amount | pays |
|----|----------|------------|--------|------|
| 1 | 1 | 1 | 20 | 0 |
| 2 | 2 | 1 | 23 | 1 |
| 4 | 1 | 2 | 14 | 1 |
| 5 | 2 | 2 | 17 | 1 |
| 6 | 1 | 3 | 10 | 1 |
| 7 | 2 | 3 | 0 | 0 |
Each row represents a contribution a payer made towards a payment. The pays field specifies whether that payer should be included in the payment.
The total cost of a payment is the sum of amount for a given payment_id. So if I wanted to see how much payment 2 cost I would run:
SELECT SUM(amount) AS sumOfPayment
FROM payers_payments
WHERE payment_id=2;
Likewise the total amount a given payer (say, payer 1) has contributed is:
SELECT SUM(amount) AS sumOfPayment
FROM payers_payments
WHERE payer_id=1;
Now, what I want to do is use the concept of fair share. Fair share is the total cost of the payment divided by the number of payers who have pay=1 for that payment. The simplest way I can express this is with a sub-query:
SELECT SUM(payers_payments.amount) / (SELECT count(*)
FROM payers_payments
WHERE pays
AND payers_payments.payment_id = 3
) AS FairShare
FROM payers_payments
WHERE payment_id=3
GROUP BY
payers_payments.payment_id;
For a given payer and payment their fair share is defined as:
SELECT IF(pays, FairShare, 0) as payerFairShare
FROM payers_payments
WHERE payer_id = 1
AND payment_id=3; --FairShare is the query as above
My question is that I want a query to get the total fair share for each payer based on the fair share of each payment and whether or not they are included in the payment. (If pays=false then their fair share is 0)
Based on the above data this is the kind of result I'm after for the 2 payers:
| payer_id | total_paid | fair_share |
|----------|------------|------------|
| 1 | 44 | 25.5 |
| 2 | 40 | 58.5 |
Is there a way to achieve this in a single query or must I do some looping of result sets?
I am agnostic on RDMS but something MySQL-like is good.

Answer 1

I would start by writing a query that works what a single share of a payment is. That is, per payment_id, the sum of all the amounts, divided by the number of people it needs to pay. That result can then be joined back to the original data.
SELECT
payers_payments.payer_id,
SUM(payers_payments.amount ) AS total_paid,
SUM(payers_payments.pays * payments.single_share) AS fair_share
FROM
payers_payments
INNER JOIN
(
SELECT
payment_id,
SUM(amount) / SUM(pays) AS single_share
FROM
payers_payments
GROUP BY
payment_id
)
AS payments
ON payers_payments.payment_id = payments.payment_id
GROUP BY
payers_payments.payer_id
It will be of benefit to have indexes on both (payment_id) and (payer_id).
It will be of benefit to have the amount field in a DECIMAL data-type, though you need to consider what you want to do with rounding. (A total payment of 10.00 needs to be divided three ways, 3.33 each and then what do you want to happen to the spare 0.01?)