select count(*),to_char(regtime,'yyyy-mm-dd hh24') from tbname group by to_char(regtime,'yyyymmddhh24')||decode(sign(to_char(regtime,'mi')-30),-1,'00','30');
select count(*),trunc(regtime) from tbname group by trunc(regtime);
select count(*),trunc(regtime,'mm') from tbname group by trunc(regtime,'mm');
select count(*),trunc(regtime,'yyyy') from tbname group by trunc(regtime,'yyyy');
select count(*) from tbname group by trunc(regtime);
select count(*) from tbname group by trunc(regtime,'mm');
select count(*) from tbname group by trunc(regtime,'yyyy');