Použijte součty přímo, protože vaše spojení pravděpodobně vytvářejí více řádků v kombinaci, kterou chcete.
Zkuste následující:
SELECT id, MAX(Total) as FinalTotal ,MAX(Payment) as FinalPayment
FROM si_invoices a
left join
(select invoice_id, sum(total) as Total from si_invoice_items group by invoice_id) b
on a.id = b.invoice_id
left join
(select ac_inv_id, sum(payment) as Payment from si_payment group by ac_inv_id) c
on c.ac_inv_id = a.id
group by id
nebo pokud je id jedinečné:
SELECT *
FROM si_invoices a
left join
(select invoice_id, sum(total) as Total from si_invoice_items group by invoice_id) b
on a.id = b.invoice_id
left join
(select ac_inv_id, sum(payment) as Payment from si_payment group by ac_inv_id) c
on c.ac_inv_id = a.id