INSERT INTO MYTAB(ID)
SELECT LEVEL FROM DUAL CONNECT BY LEVEL<=100;

SELECT DBMS_METADATA.GET_DDL('TABLE','ИМЯ ТАБЛИЦЫ','ИМЯ ВЛАДЕЛЬЦА ТАБЛИЦЫ') FROM DUAL;

DELETE
FROM
  ALL_SNAPSHOTS
WHERE
  SNAPSHOT_TIME                       <=SYSDATE-12/24
  AND (SNAPSHOT_TIME, ID_CHANNEL) NOT IN
  (
    SELECT MAX(SNAPSHOT_TIME), ID_CHANNEL FROM ALL_SNAPSHOTS GROUP BY ID_CHANNEL
  );