Calculate Duration

Here is the example query to calculate duration in hour :

select (to_date('2016-12-06 22:00', 'YYYY-MM-DD hh24:mi') - to_date('2016-12-06 19:30', 'YYYY-MM-DD hh24:mi')) * 24 duration_hours 
       from dual;

Result in minute :

select (to_date('2016-12-06 22:00', 'YYYY-MM-DD hh24:mi') - to_date('2016-12-06 19:30', 'YYYY-MM-DD hh24:mi')) * 24 * 60 duration_minutes 
       from dual;

Result in second :

select (to_date('2016-12-06 22:00', 'YYYY-MM-DD hh24:mi') - to_date('2016-12-06 19:30', 'YYYY-MM-DD hh24:mi')) * 24 * 60 * 60 duration_seconds 
       from dual;