create or replace trigger a_replica
before insert or update on a
for each row
begin
DELETE FROM b;
INSERT INTO  b
SELECT * FROM A;
END;

CREATE OR REPLACE TRIGGER A_replica
AFTER INSERT ON A FOR EACH ROW BEGIN
IF ????(--create a sequence here?,how to identify new columns)THEN
INSERT INTO B 
SELECT * FROM A ;
END IF;
EXCEPTION
WHEN OTHERS THEN
RAISE_APPLICATION_ERROR (-20001, sqlcode() || ': ' ||sqlerrm());
END;
/