Oracle masking data using a trigger - sql

Question

Im trying to create a process that masks data. When I create the trigger I'm getting the error.
ORA-04072: invalid trigger type
I'm unsure why and was hoping someone can explain what the problem is and how to fix it.
The end result is when a user queries cards they should see the masked data and WHEN they query CARDS_TBL they should see all the data (unmasked)
Original implementation
CREATE TABLE CARDS (
CARD_ID NUMBER
GENERATED BY DEFAULT AS IDENTITY,
CARD_STR VARCHAR2(16) NOT NULL,
PRIMARY KEY (CARD_ID)
);
INSERT INTO CARDS(CARD_STR) VALUES('4024007187788590');
INSERT INTO CARDS(CARD_STR) VALUES('5432223398564536');
INSERT INTO CARDS(CARD_STR) VALUES('5430445512530934');
INSERT INTO CARDS(CARD_STR) VALUES('4020156755227854');
INSERT INTO CARDS(CARD_STR) VALUES('5431248766892318');
CREATE OR REPLACE VIEW CARDS_V AS
SELECT
CARD_ID,
REGEXP_REPLACE(CARD_STR, '(^\d{3})(.*)(\d{4}$)', '\1**********\3') AS CARD_STR
FROM CARDS;
CREATE OR REPLACE TRIGGER CARDS_TRG_INSERT INSTEAD OF
INSERT ON CARDS_V
FOR EACH ROW
BEGIN
INSERT INTO CARDS (CARD_STR) VALUES (:NEW.CARD_STR);
END;
INSERT INTO CARDS_V (CARD_STR) VALUES ('4011589733550908');
CREATE OR REPLACE TRIGGER CARDS_TRG_UPDATE INSTEAD OF
UPDATE ON CARDS_V
FOR EACH ROW
BEGIN
UPDATE CARDS
SET CARD_STR = :NEW.CARD_STR
WHERE CARD_ID = :OLD.CARD_ID;
END;
CREATE TABLE CARDS_TBL (
CARD_ID NUMBER
GENERATED BY DEFAULT AS IDENTITY,
CARD_STR VARCHAR2(16) NOT NULL,
PRIMARY KEY (CARD_ID)
);
INSERT INTO CARDS_TBL(CARD_STR) VALUES('4024007187788590');
INSERT INTO CARDS_TBL(CARD_STR) VALUES('5432223398564536');
INSERT INTO CARDS_TBL(CARD_STR) VALUES('5430445512530934');
INSERT INTO CARDS_TBL(CARD_STR) VALUES('4020156755227854');
INSERT INTO CARDS_TBL(CARD_STR) VALUES('5431248766892318');
CREATE OR REPLACE VIEW CARDS AS
SELECT
CARD_ID,
REGEXP_REPLACE(CARD_STR, '(^\d{3})(.*)(\d{4}$)', '\1**********\3') AS CARD_STR
FROM CARDS_TBL;
CREATE OR REPLACE TRIGGER CARDS_TBL_TRG_UPDATE BEFORE UPDATE ON CARDS_TBL
FOR EACH ROW
BEGIN
UPDATE CARDS_TBL
SET CARD_STR = :NEW.CARD_STR
WHERE CARD_ID = :OLD.CARD_ID;
END;
/
CREATE OR REPLACE TRIGGER CARDS_TBL_TRG_INSERT BEFORE INSERT ON CARDS
FOR EACH ROW
BEGIN
INSERT INTO CARDS_TBL (CARD_STR) VALUES (:NEW.CARD_STR);
END;
INSERT INTO CARDS_TBL (CARD_STR) VALUES ('2222333344445555');
SELECT * FROM CARDS_TBL;
UPDATE CARDS_TBL
SET CARD_STR = '2222333344445566'
WHERE CARD_ID = 6;
/
SELECT * FROM CARDS;

Answer 1

In this particular example, at least, the trigger CARDS_TBL_TRG_UPDATE doesn't do anything (except raise a MUTATING TABLE exception) and can be dispensed with. Get rid of it and your example runs as expected. See this db<>fiddle