--insert records
insert into userlogin_his(usrname,logintime) values('test',to_timestamp(to_char(sysdate,'YYYY-MM-DD HH24:MI:SS'),'YYYY-MM-DD HH24:MI:SS'));
insert into userlogin_his(usrname,logintime) values('test1',to_timestamp(to_char(systimestamp,'YYYY-MM-DD HH24:MI:SS.FF'),'YYYY-MM-DD HH24:MI:SS.FF'));
insert into userlogin_his(usrname,logintime) values('test2',systimestamp);
insert into userlogin_his(usrname,logintime) values('test3',sysdate);

--query records
select usrname,to_char(logintime,'YYYY-MM-DD HH24:MI:SS.FF')  from userlogin_his;

USRNAME    TO_CHAR(LOGINTIME,'YYYY-MM-DDHH24:MI:SS.FF')

test    2017-03-30 18:09:59.000000

test1    2017-03-30 18:13:27.505000

test2    2017-03-30 18:28:16.521000

test3    2017-03-30 18:29:48.000000

從以上語句和執行結果來看，第4行程式碼是最簡潔明瞭的。推薦採用。