Get first Data after Oracle sort __oracle

First of all want to say, in fact very simple

SELECT * FROM (SELECT * to teststudent ORDER BY id DESC) where rownum=1;

Look at the SQL statement above

Execute the parentheses inside the query out all the students, and in descending order, in the execution outside of the rownum=1 get the first statement.

In the next statement, why can't we get the first piece of data we want?

SELECT * from Teststudent where rownum=1 the order by id DESC;

The main is the order is not right, select * from Teststudent where rownum=1 execute here, there's only one piece of data. ORDER BY id desc in descending order there is no meaning to have a single statement