Puedes hacer
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
Tenga en cuenta la restricción invoices_elements_enable = 1
está en ON
cláusula para evitar convertir la consulta en una inner join
.