Nakonec jsem to udělal celé takto:

SELECT inza.firma, indo.nazevFirmy, doin.Cobjednavky, inmn.mnakladky, inmv.mvykladky, obin.datumNakladky, obin.datumVykladky, obin.cena, doin.cena AS Expr1,
obin.cena - doin.cena AS Expr2
FROM info_dopravce AS indo INNER JOIN
dopravce_info AS doin ON indo.id = doin.dopravce INNER JOIN
objednavka_info AS obin ON doin.Cobjednavky = obin.Cobjednavky INNER JOIN
info_zakaznik AS inza ON obin.zakaznik = inza.id INNER JOIN
info_mvykladky AS inmv ON obin.mistoVykladky = inmv.id INNER JOIN
info_mnakladky AS inmn ON obin.mistoNakladky = inmn.id

a děkuji moc už tomu rozumím.