MySQL slutter sig kun til udvalgte rækker

Du kan gøre

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  

Bemærk begrænsningen invoices_elements_enable = 1 er i ON klausul for at undgå at konvertere forespørgslen til en inner join .