CREATE OR REPLACE PROCEDURE P_ADD_STATION(
   s_HEAD IN STATION.ID%TYPE,
   s_TAIL IN STATION.ID%TYPE,
   s_NAME IN STATION.NAME%TYPE
   s_ID OUT STATION.ID%TYPE
   ) IS

   t_ID TRANSFER.ID%TYPE;
BEGIN

  IF (TRIM(p_NAME) IS NULL) THEN
    RAISE_APPLICATION_ERROR(-20101, 'Name should be not null');
  END IF;

  IF (s_HEAD IS NULL AND s_TAIL IS NULL) THEN
    RAISE_APPLICATION_ERROR(-20102, 'New station siblings were not specified');
  END IF;

  IF (s_TAIL IS NULL) THEN
    RAISE_APPLICATION_ERROR(-20103, 'New station must be either after or in between existing stations');
  END IF;

  s_ID := seq_STATION.NEXTVAL;
  INSERT INTO STATION(ID,NAME) VALUES (s_ID, s_NAME);

  t_ID := seq_TRANSFER.NEXTVAL;
    INSERT INTO TRANSFER(ID,FROM,TO) VALUES (t_ID, s_TAIL, s_ID);

  IF (s_HEAD IS NOT NULL) THEN
    t_ID := seq_TRANSFER.NEXTVAL;
    INSERT INTO TRANSFER(ID,FROM,TO) VALUES (t_ID, s_ID, s_HEAD);
  END IF;

END P_ADD_STATION;