Cannot to create trigger properly - sql

Question

Oracle apex, PL_Sql.
This is my trigger:
CREATE OR REPLACE TRIGGER NoMoreThanOneHorse
BEFORE INSERT OR UPDATE OF Jockey_ID
ON Horses
FOR EACH ROW
DECLARE
NumOfHorsesForJockey NUMBER(4);
BEGIN
SELECT COUNT(*) INTO NumOfHorsesForJockey FROM Horses
WHERE Jockey_ID = :NEW.Jockey_ID AND Horse_ID <> :NEW.Horse_ID;
IF NumOfHorsesForJockey > 0
THEN RAISE_APPLICATION_ERROR (-20445, 'Нельзя закрепить за лошадью уже занятого жокея!');
END IF;
END NoMoreThanOneHorse;
but every time i try to create it i have this error :ORA-24344: success with compilation error ORA-06512. I suppose it's because bad syntax

Answer 1

I don't know what the compile-time error might be - it compiles fine for me. But at run-time you'll probably get an `ORA-04091 Table HORSES is mutating, trigger cannot see it" error. You can't fetch from HORSES in a ROW trigger which is defined on the HORSES table.
What you need to do is to change this to an AFTER STATEMENT trigger, which you do by leaving off the FOR EACH ROW in the trigger definition, and changing the trigger point from BEFORE to AFTER. The problem is that in a statement trigger you don't have access to the :NEW or :OLD data, but you don't really need it. A statement trigger is invoked only once for each statement which is executed. So your trigger should look something like:
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;
You really don't care which jockey has been assigned too many horses, only that there is a jockey who's been put on more than one horse.
db<>fiddle here