Андрей

with inp as (
select 1 id from dual union all
select 2 from dual union all
select 3 from dual union all
select 4 from dual union all
select 5 from dual union all
select 6 from dual)

SELECT
    id
FROM
    inp
ORDER BY
    abs( (
        SELECT
            AVG(id)
        FROM
            inp
    ) - id),
    id;

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
  );