PRIOR in SELECT list
<p>I can't understand what it adds to the result of the query. From the book that I'm learning: </p>

<blockquote>
  <p>If you prefix a column name with PRIOR in the
  select list (SELECT PRIOR EMPLOYEE_ID, ...), you specify the “prior” row’s value.</p>
</blockquote>

<pre><code>SELECT PRIOR EMPLOYEE_ID, MANAGER_ID, LPAD(' ', LEVEL * 2) || EMPLOYEES.JOB_ID
  FROM EMPLOYEES
  START WITH EMPLOYEE_ID = 100
  CONNECT BY PRIOR EMPLOYEE_ID =  MANAGER_ID;
</code></pre>

<p>The only difference I see, is that it adds a <code>NULL</code> value in the first row and increments IDs of employees by 1.</p>