fetch unique invoices though several reminders (one-to-many relation)? - postgresql-9.1

Question

Here is a simplified sqlfiddle as basis for my question:
http://sqlfiddle.com/#!15/c114a/7
The first invoice have two reminders sent from two other dates:
I want to get all invoice that should be reminded - once. Now it duplicates by reminders.
I want only the last reminder
SELECT inv_id, inv_due, rem_type
FROM inv
LEFT JOIN remrec ON remrec_inv = inv_id
LEFT JOIN rem ON rem_id = remrec_rem
WHERE inv_due < '2015-01-02'
And I know that i can compare and get the latest date from invoice and all reminders using greatest(inv_due,rem1,rem2,rem3):
SELECT inv_id,rem_type,greatest(inv_due,
(CASE WHEN rem_date<'2015-01-02' AND rem_type='1' THEN rem_date ELSE NULL END),
(CASE WHEN rem_date<'2015-01-02' AND rem_type='2' THEN rem_date ELSE NULL END),
(CASE WHEN rem_date<'2015-01-02' AND rem_type='3' THEN rem_date ELSE NULL END)) as max
FROM inv
LEFT JOIN remrec ON inv_id=remrec_inv
LEFT JOIN rem ON rem_id=remrec_id
WHERE inv_due < '2015-01-02'
So each reminder could consists of several invoices. Hence the rem and remrec.
The question is: How do I get only the unique invoices that should be reminded?
Thank you in advance for any clue.

Answer 1

I think I found a solution:
SELECT inv_id, inv_due, max(rem_type)
FROM inv
LEFT JOIN remrec ON inv_id=remrec_inv
LEFT JOIN rem ON rem_id=remrec_rem
WHERE greatest(inv_due,rem_date) < '2015-01-05'
GROUP BY inv_id, inv_due
Edit: Works better with several reminders:
SELECT DISTINCT inv_id, inv_due
FROM inv
LEFT JOIN remrec ON inv_id=remrec_inv
LEFT JOIN rem ON rem_id=remrec_rem
WHERE inv_due<'2015-01-05'
GROUP BY inv_id,inv_due
HAVING greatest(max(rem_date),inv_due)<'2015-01-05'
FWIW.