with t as
(select to_timestamp('2015-01-01 11:13:15.023456',
'yyyy-mm-dd hh24:mi:ss.ff9') t1,
to_timestamp('2015-01-01 12:13:15.123556',
'yyyy-mm-dd hh24:mi:ss.ff9') t2
from dual)
select abs((trunc(t1 - 0, 'mi') - trunc((t2 - 0), 'mi')) * 24 * 60 * 60 +
extract(second from t1 - t2))
from t
输出结果： 3600.1001