Você pode fazer
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
Observe a restrição
invoices_elements_enable = 1 está em ON cláusula para evitar converter a consulta em uma inner join . 
