oracle定时任务半小时执行一次怎么设置

<code class="SQL">BEGIN
  DBMS_SCHEDULER.CREATE_SCHEDULE(
    schedule_name =&gt; \'JOB_SCHEDULE\',
    start_date =&gt; SYSDATE,
    repeat_interval =&gt; \'FREQ=MINUTELY;INTERVAL=30\',
    end_date =&gt; NULL
  );
END;
/