How to use Oracle ORDER BY and ROWNUM correctly?

Since Oracle 12c we now have row limiting clauses which do exactly this.

SELECT *
FROM raceway_input_labo 
ORDER BY t_stamp DESC
FETCH FIRST ROW ONLY

Or many alternatives for different scenarios (first n rows, tie handling, etc.).

The where statement gets executed before the order by. So, your desired query is saying "take the first row and then order it by t_stamp desc". And that is not what you intend.

The subquery method is the proper method for doing this in Oracle.

If you want a version that works in both servers, you can use:

select ril.*
from (select ril.*, row_number() over (order by t_stamp desc) as seqnum
      from raceway_input_labo ril
     ) ril
where seqnum = 1

The outer * will return "1" in the last column. You would need to list the columns individually to avoid this.

Use ROW_NUMBER() instead. ROWNUM is a pseudocolumn and ROW_NUMBER() is a function. You can read about difference between them and see the difference in output of below queries:

SELECT * FROM (SELECT rownum, deptno, ename
           FROM scott.emp
        ORDER BY deptno
       )
 WHERE rownum <= 3
 /

ROWNUM    DEPTNO    ENAME
---------------------------
 7        10    CLARK
 14       10    MILLER
 9        10    KING


 SELECT * FROM 
 (
  SELECT deptno, ename
       , ROW_NUMBER() OVER (ORDER BY deptno) rno
  FROM scott.emp
 ORDER BY deptno
 )
WHERE rno <= 3
/

DEPTNO    ENAME    RNO
-------------------------
10    CLARK        1
10    MILLER       2
10    KING         3