title: GROUP BY combined with ORDER BY
body: <blockquote>
    <p>The GROUP BY clause groups the rows, but it does not necessarily sort the results in any particular order. To change the order, use the ORDER BY clause, which follows the GROUP BY clause. <strong>The columns used in the ORDER BY clause must appear in the SELECT list, which is unlike the normal use of ORDER BY.</strong> [Oracle by Example, fourth Edition, page 274]</p>
  </blockquote>
  
  <p>Why is that? Why does using GROUP BY influence the required columns in the SELECT clause?</p>
  
  <p>Also, in the case where I do not use GROUP BY: Why would I want to ORDER BY some columns but then select only a subset of the columns?</p>