(SELECT 1 AS sort_col, nazev, kategorie, dodavatel, cena FROM zbozi WHERE zobrazit = "1" AND kategorie = "pečivo") 

UNION

(SELECT 2, nazev, kategorie, dodavatel, cena FROM zbozi WHERE zobrazit = "1" AND kategorie != "pečivo")
ORDER BY sort_col, kategorie, dodavatel, cena

(SELECT * FROM zbozi WHERE zobrazit = "1" AND kategorie = "pečivo" ORDER BY dodavatel, cena) 

UNION

(SELECT * FROM zbozi WHERE zobrazit = "1" AND kategorie != "pečivo" ORDER BY kategorie, dodavatel, cena)

select * from zbozi order by if (kategorie='pecivo','',kategorie), dodavatel, cena;

select * from zbozi order by

case kategorie

  when 'pecivo' then 1

  when 'maso'   then 2

  when 'limo'   then 3

end

, dodavatel, cena;