CREATE OR REPLACE FUNCTION change_quantity_after_adding_new_expire_date()
  RETURNS trigger AS
$$
  DECLARE
       temp_quantity         integer;
       old_quantity          integer;

       quanity_difference     integer;


BEGIN
    old_quantity = (SELECT quantity FROM medicines WHERE medicines.id= NEW.medicine_id);

    IF (TG_OP = 'DELETE') THEN
        quanity_difference =  -1*OLD.quantity_piece;
    ELSEIF (TG_OP = 'UPDATE') THEN
        quanity_difference= NEW.quantity_piece - OLD.quantity_piece;
    ELSEIF (TG_OP = 'INSERT') THEN
        quanity_difference = NEW.quantity_piece;
    END IF;

        temp_quantity =old_quantity+quanity_difference;
    UPDATE medicines SET quantity = temp_quantity WHERE  medicines.id= NEW.medicine_id;
    RETURN OLD;
END
$$
LANGUAGE 'plpgsql';