col LOG_HOUR format a12
select to_char(first_time,'YYYY-mm-dd')       LOG_DATE, to_char(first_time,'HH24')       LOG_HOUR, count(*)                         SWITCHES  from v$loghist  group by to_char(first_time,'YYYY-mm-dd') , to_char(first_time,'HH24')  order by 1,2;