oracle语句两张表相减，看最后是否有误差

select total_seconds1,total_seconds2,total_seconds1-total_seconds2 AS gap from 
(select (hours+minutes+seconds) AS total_seconds1 from (
　　select
　　to_char(sysdate,(‘HH24‘))*60*60 AS hours,
　　to_char(sysdate,(‘mi‘))*60 AS minutes,
　　to_char(sysdate,‘ss‘) AS seconds
　　from dual
　　)),
(select (hours+minutes+seconds) AS total_seconds2 from (
　　select
　　to_char(sysdate,(‘HH24‘))*60*60 AS hours,
　　to_char(sysdate,(‘mi‘))*60 AS minutes,
　　to_char(sysdate,‘ss‘) AS seconds
　　from(
　　　　select 
　　　　to_char(f_quotation_time,‘HH24:mi:ss‘) 
　　　　from A tq,B tc where 
　　　　tq.f_contract_id = (select f_id from B where f_symbol = ‘AG‘) and tc.f_symbol = ‘AG‘ )
　　));