MYSql linking data from 2 tables trigger - calculating value using one and putting into another

I have a problem in a trigger that will calculate the data taking values from one table row and multiplying it by another from different table and putting the value in. I come up with such a trigger:

DELIMITER $$

CREATE TRIGGER liczy_zarobek_koszyk
    BEFORE INSERT
    ON koszyk
FOR EACH ROW BEGIN
INSERT FROM koszyk SET a = NEW.koszyk_ilosc;    - ERROR
INSERT FROM produkty SET b = NEW.produkt_zarobek;   -ERROR
UPDATE koszyk SET koszyk_zarobek = a * b;
END;
$$

but it seems to have errors. I would appreciate any advise in fixing the problem :)

TABLES:

CREATE TABLE `rowerowy`.`koszyk` (

  `koszyk_id` INT NOT NULL AUTO_INCREMENT ,

  `koszyk_ilosc` INT(10) NULL ,

  `koszyk_data` DATETIME NOT NULL DEFAULT NOW() , -- przy kazdym dopisaniu dopisuje date dopisania z teraz:)

  `koszyk_zarobek` DECIMAL NULL,

  `produkt_id` INT NOT ,

  PRIMARY KEY (`koszyk_id`),

  CONSTRAINT `produkt_id`

    FOREIGN KEY (`produkt_id` )

    REFERENCES `rowerowy`.`produkty` (`produkt_id` )

    ON DELETE NO ACTION

    ON UPDATE NO ACTION
);

CREATE  TABLE `rowerowy`.`produkty` (

  `produkt_id` INT NOT NULL AUTO_INCREMENT ,

  `produkt_producent` VARCHAR(45) NULL ,

  `produkt_model` VARCHAR(45) NULL ,

  `produkt_rok` INT(4) NULL ,

  `produkt_cena_kupna` DECIMAL NULL ,

  `produkt_cena` DECIMAL NULL ,

  `produkt_zarobek` DECIMAL NULL ,

  `produkt_ilosc` INT(10) NULL ,

  `kategoria_id` INT NOT NULL ,

  `dostawcy_id` INT NOT NULL ,

  `koszyk_id` INT NOT NULL ,

  PRIMARY KEY (`produkt_id`) ,

  INDEX `koszyk_id` (`koszyk_id` ASC) ,

  INDEX `dostawcy_id` (`dostawcy_id` ASC) ,

  INDEX `kategoria_id` (`kategoria_id` ASC) ,

  CONSTRAINT `koszyk_id`

    FOREIGN KEY (`koszyk_id` )

    REFERENCES `rowerowy`.`koszyk` (`koszyk_id` )

    ON DELETE NO ACTION

    ON UPDATE NO ACTION,

  CONSTRAINT `dostawcy_id`

    FOREIGN KEY (`dostawcy_id` )

    REFERENCES `rowerowy`.`dostawcy` (`dostawcy_id` )

    ON DELETE NO ACTION

    ON UPDATE NO ACTION,

  CONSTRAINT `kategoria_id`

    FOREIGN KEY (`kategoria_id` )

    REFERENCES `rowerowy`.`kategoria` (`kategoria_id` )

    ON DELETE NO ACTION

    ON UPDATE NO ACTION
);

Tags: MySQL, Table, Trigger