Zkuste tento dotaz:
SELECT `cus_id` as CustId, (SELECT `total_due` FROM invoice WHERE cus_id = CustId ORDER BY `inv_id` DESC LIMIT 1) as total_due FROM invoice GROUP BY cus_id
Zkuste tento dotaz:
SELECT `cus_id` as CustId, (SELECT `total_due` FROM invoice WHERE cus_id = CustId ORDER BY `inv_id` DESC LIMIT 1) as total_due FROM invoice GROUP BY cus_id