SELECT COALESCE(IDZakaz, '') FROM ...
SELECT * FROM rozpocty_rozpocty WHERE COALESCE(IDzakaz, '') = ''

Nepoužívat konrstrukce subselectů "not in (select …)", ale predikát EXISTS.

SELECT * FROM sklad_vc vc
  WHERE 
    NOT EXISTS (SELECT * FROM sklad_prijemky prijemky WHERE prijemky.idprijemka=vc.idprijemka)