title: PRIOR in SELECT list
body: <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>