方法1: from_unixtime+ unix_timestamp
--20171205轉成2017-12-05 
select from_unixtime(unix_timestamp('20171205','yyyymmdd'),'yyyy-mm-dd') from dual;

--2017-12-05轉成20171205
select from_unixtime(unix_timestamp('2017-12-05','yyyy-mm-dd'),'yyyymmdd') from dual;

方法2: substr + concat
--20171205轉成2017-12-05 
select concat(substr('20171205'
 
,1,4),'-',substr('20171205',5,2),'-',substr('20171205',7,2)) from dual;

--2017-12-05轉成20171205
select concat(substr('2017-12-05',1,4),substr('2017-12-05',6,2),substr('2017-12-05',9,2)) from dual;