Calculate payment date for each invoice - sql

Question

Please consider the following table transaction: a company regularly sends invoices to their customers that are part of the same order. The companies' clients will often pay only once per so many weeks.
(trans_date in format yyyy-mm-dd)
id order_id trans_type trans_date trans_amount
----------------------------------------------------------
1 1 invoice 2017-01-10 100
2 1 invoice 2017-05-23 150
3 1 invoice 2017-05-28 200
4 2 invoice 2017-03-01 700
5 2 payment 2017-06-16 700
6 1 payment 2017-10-12 450
7 3 invoice 2017-06-24 199
The company would like to see on what date each invoice was paid for. For example: invoice (id) 1 (part of order_id=1 group) was sent on 2017-01-10 and paid on 2017-10-12 (id=6). Invoice with id=7 has not been paid at all.
The desired output would be the payment date for each invoice (payment_date):
id order_id trans_type trans_date trans_amount payment_date
--------------------------------------------------------------------------
1 1 invoice 2017-01-10 100 2017-10-12
2 1 invoice 2017-05-23 150 2017-10-12
3 1 invoice 2017-05-28 200 2017-10-12
4 2 invoice 2017-03-01 700 2017-06-16
5 2 payment 2017-06-16 700
6 1 payment 2017-10-12 450
7 3 invoice 2017-06-24 199
For transactions 5, 6 and 7, the payment_date is empty because it is either a payment (id=5 and 6) or an unpaid invoice (id=7).
I don't understand how I should solve this issue. In combination with regular scripting, I would get the whole set and loop through it to find each payment. But how can this be solved in SQL only?
Any help would be greatly appreciated!

Answer 1

Did you try a simple left join?
Below code is standard SQL.
Select a.id , a.order_id, a.trans_type, a.trans_date, a.trans_amount, isnull(b.trans_date, '') As payment_date
From transaction a
Left join transaction b
On a.order_id = b.order_id
And a.trans_type = 'invoice'
And b.trans_type = 'payment'

Answer 2

You can do a cumulative sum of payments and invoices and get the first date when the payment total meets or exceeds the invoice total:
with ip as (
select ip.*,
sum(case when ip.trans_type = 'invoice' then ip.trans_amount else 0 end) over (order by ip.trans_date) as running_invoice,
sum(case when ip.trans_type = 'payment' then ip.trans_amount else 0 end) over (order by ip.trans_date) as running_payment,
from invoicepayments i
)
select ip.*,
(select min(ip2.trans_date)
from ip ip2
where ip2.running_payment >= ip.running_invoice and
ip.trans_type = 'invoice'
) as payment_date
from ip;