Since you do not know if it's a firm or a person, you could use left outer join on both, like this:
SELECT PR.id_projektu, PR.popis, ZAD.id_zadavatela, FI.nazov_firmy, SO.meno
FROM Projekt PR
JOIN Zamestnanec ZAM ON PR.manazer = ZAM.osobne_cislo
JOIN Zadavatel ZAD ON PR.zadavatel = ZAD.id_zadavatela
LEFT OUTER JOIN Firma FI ON ZAD.id_zadavatela = FI.id_firmy
LEFT OUTER JOIN Sukromna_osoba SO ON ZAD.id_zadavatela = SO.id_sukromnej_osoby
WHERE ZAM.meno = 'Jan Novák'
One of the two result columns, nazov_firmy
or meno
, will be NULL
.