Zkuste toto:
DELIMITER $$
CREATE TRIGGER `yourDB_polozky`
BEFORE INSERT
ON `yourDB`.`polozky`
FOR EACH ROW
BEGIN
DECLARE vat_rate float;
SELECT DPH
INTO dph_rate
FROM `yourDB`.`dph`
WHERE id = new.dph_id;
SET new.cena = (1 + dph_rate) * (new.cena_za_kus * new.mnozstvi);
END
$$