81. 求时间差（08:30-10:00）

select ROUND(TO_NUMBER(to_date('10:00','hh24:mi') - to_date('08:30','hh24:mi')) * 24 * 60) from dual; 

 

select ROUND(TO_NUMBER(TO_DATE('09:00:00','HH24:MI:SS') - to_date('08:00:00','HH24:MI:SS')) * 24 * 60) from dual;