select (COMPLETED_DATE - START_DATE) as diff_days
it will be a real number like 1.23423 meaning 1 day and 0.23423 x 24 = 5.62152 hours
maning 5 hours + 0.62152 * 60 = 37.2912 minutes
meaning 37 minutes and 0.2912 * 60 = 17.472 seconds ;)
if you multiply the difference with 24*60*60, you will end up seconds
select (COMPLETED_DATE - START_DATE)* (24*60*60) as diff_seconds