Oracle 21c

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CREATE TABLE test (
  id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  data VARCHAR(255)
);
INSERT INTO test(id,data) VALUES(3,'something');
1 rows affected

INSERT INTO test(id,data) VALUES(6,'something else');
1 rows affected

INSERT INTO test(data) VALUES('whatever');
1 rows affected

INSERT INTO test(data) VALUES('stuff');
1 rows affected

ALTER TABLE test MODIFY ID GENERATED BY DEFAULT AS IDENTITY (START WITH LIMIT VALUE);
INSERT INTO test(data) VALUES('etc');
1 rows affected

SELECT * FROM test;


ID
DATA


3
something

6
something else

1
whatever

2
stuff

7
etc