Je kunt doen
SELECT SUM(netto)
FROM invoices
LEFT JOIN invoices_elements
ON invoices_elements_code_invoices_id = invoices_id
AND invoices_elements_enable = 1
WHERE invoices_enable = 1
GROUP BY invoices_elements_code_invoices_id
Let op de beperking invoices_elements_enable = 1
staat in de ON
clausule om te voorkomen dat de query wordt omgezet in een inner join
.