Oracle 21c

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CREATE TABLE test_guid(
  id   INT,
  guid RAW(16)
)
CREATE OR REPLACE TRIGGER "XDA.GUID"
  BEFORE INSERT ON TEST_GUID
  FOR EACH ROW
  WHEN (NEW.guid IS NULL)
BEGIN
  :NEW.guid := sys_guid();
END;
/
SELECT * FROM USER_ERRORS;
INSERT INTO test_guid(id, guid) VALUES(1,HEXTORAW('F7DE3340860C0A05E053182BA8C04D74'))
1 rows affected

INSERT INTO test_guid(id) VALUES(2)
1 rows affected

INSERT INTO test_guid(id, guid) VALUES (3, NULL);
1 rows affected

INSERT INTO test_guid(id, guid) VALUES (4, NULL);
1 rows affected

SELECT *
  FROM test_guid


ID
GUID


1
0xF7DE3340860C0A05E053182BA8C04D74

2
0xF7DF257DDA5D0D51E053182BA8C0FE20

3
0xF7DF257DDA5E0D51E053182BA8C0FE20

4
0xF7DF257DDA5F0D51E053182BA8C0FE20

ID	GUID
1	0xF7DE3340860C0A05E053182BA8C04D74
2	0xF7DF257DDA5D0D51E053182BA8C0FE20
3	0xF7DF257DDA5E0D51E053182BA8C0FE20
4	0xF7DF257DDA5F0D51E053182BA8C0FE20