Oracle 11g Release 2

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CREATE TABLE HORSES (HORSE_ID  NUMBER PRIMARY KEY,
                     JOCKEY_ID  NUMBER);
    CREATE OR REPLACE TRIGGER NoMoreThanOneHorse
        AFTER INSERT OR UPDATE OF Jockey_ID
        ON Horses
    DECLARE
        nMaxHorses  NUMBER;
    BEGIN
        SELECT MAX(HORSE_COUNT)
          INTO nMaxHorses
          FROM (SELECT JOCKEY_ID, COUNT(*) AS HORSE_COUNT
                  FROM Horses
                  GROUP BY Jockey_ID);

        IF nMaxHorses > 1 THEN
          RAISE_APPLICATION_ERROR (-20445, 'Нельзя закрепить за лошадью уже занятого жокея!');
        END IF;
    END NoMoreThanOneHorse;
/
INSERT INTO HORSES (HORSE_ID, JOCKEY_ID)  VALUES (1, 1);
1 rows affected

INSERT INTO HORSES (HORSE_ID, JOCKEY_ID) VALUES (2, 1);
ORA-20445: ???????????? ?????????????????? ???? ?????????????? ?????? ???????????????? ??????????!
ORA-06512: at "FIDDLE_IWMEWJQSSMEGUCLJIEQF.NOMORETHANONEHORSE", line 11
ORA-04088: error during execution of trigger 'FIDDLE_IWMEWJQSSMEGUCLJIEQF.NOMORETHANONEHORSE'