CREATE OR REPLACE TRIGGER jockey_check BEFORE INSERT OR UPDATE ON horse_races
for each row
DECLARE horse_jockey_id number;
first_name_jockey VARCHAR2(100);
first_name_owner VARCHAR2(100);
horse_id number;
owner_id number;
BEGIN
  SELECT horse_races.horse_jockey_id INTO horse_jockey_id
    FROM horse_races
    WHERE
      horse_jockey_id = :new.horse_jockey_id;

  SELECT jockeys.first_name INTO first_name_jockey
    FROM jockeys
    WHERE
      jockeys.id = horse_jockey_id;

  SELECT horse_jockeys.horse_id INTO horse_id
    FROM horse_jockeys
    WHERE
      horse_jockeys.jockey_id = horse_jockey_id;

  SELECT horse_owners.owner_id INTO owner_id
    FROM horse_owners
    WHERE
      horse_owners.horse_id = horse_id;

  SELECT first_name INTO first_name_owner
    FROM owners
    WHERE
      owners.id = owner_id;

  IF (
    first_name_owner = first_name_jockey
  )
  THEN
    raise_application_error(
      -20007,
      'The name of jockey cannot be the same as owner.'
    );
  END IF;
END;