How to retrieve unpaid amount of each invoice for finance charges? - sql

Question

I am working on implementing finance charges, but don't know how to retrieve the unpaid amount of each invoice. I am not new to SQL, however this problem has me stumped. Forgive me if I just didn't search properly.
So, let us say I have an invoice table:
id | amount
----+--------------
1 | 50.00
2 | 50.00
3 | 50.00
4 | 50.00
5 | 50.00
And a payments table:
amount
--------------
50.00
25.00
The result set should be this:
invoice_id | unpaid_amount
------------+--------------
2 | 25.00
3 | 50.00
4 | 50.00
5 | 50.00
Of course, there is quite a bit more to add to implement finance charges, but I think I can get the rest.
Edit: Sorry, an oversight of mine. The id's are not related. Removed payment id column.
Edit 2: These are fictual numbers, the real life numbers will be anything, so no matches can be made on the amounts.
Edit 3: And here I have created a SQL Fiddle to show what I have so far, based on #GordonLinoff second answer. I would appreciate a cleaner approach than the kludgy SQL I concocted.

Answer 1

I think you want a join:
select i.id as invoice_id,
(i.amount - coalesce(p.amount, 0)) as net_amount
from invoice i left join
payment p
on i.id = p.id;
EDIT:
Or, you may want:
select i.*,
(case when sum(i.amount) over (order by i.id) < p.amount
then i.amount
else greatest(p.amount - sum(i.amount) over (order by i.id) + i.amount, 0)
end) as amount_paid
from invoice i cross join
(select sum(amount) as amount
from payment
) p;
Here is a db<>fiddle.

Answer 2

You can do by using LEFT JOIN and subtract amounts from both table. COALSECE is here to prevent you from getting NULL when there is no payment.
select
i.id as invoice_id
,i.amount - coalesce(p.amount,0) as amount
from invoice i
left join payments p
on p.id = i.id
If in your payments talbe can be more than one payment for an invoice you should group payments before joining it to invoice.
select
i.id as invoice_id
,i.amount - coalesce(p.amount,0) as amount
from invoice i
left join (select id, sum(amount) as amount from payments group by id) p
on p.id = i.id