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)

SELECT 
  CASE 
    WHEN (FIELD = 1) THEN 'x' 
    WHEN (FIELD = 2) THEN 'y' 
    ELSE 'z' 
  END 
FROM ...

SELECT SUBSTRING(FIELD FROM 1 FOR 5) FROM ...