title: oracle sql group count
body: <pre><code>SELECT a,b,count(*)
  FROM t
  GROUP BY rollup(a,b)
  </code></pre>
  
  <p>result:</p>
  
  <pre><code>a1, b1, 10
  a1, b2, 90
  a1,   , 100
  </code></pre>
  
  <p>i need:</p>
  
  <pre><code>a1, b1, 10, 100
  a1, b2, 90, 100
  </code></pre>
  
  <p>how?</p>