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;