title: how do I select AVG of multiple columns on a single row
body: <p>How can I select average of multiple columns?</p>
  
  <p>Suppose I have some data like: </p>
  
  <pre><code>X   Y    Z
  -------------
  6   3    3
  5   5    NULL
  4   5    6
  11  7    8
  </code></pre>
  
  <p>I want to get something like</p>
  
  <pre><code>AVG
  -------------
  4
  5
  5
  8.66666667
  </code></pre>
  
  <p>I tried <code>select avg(x, y, z) from table</code> </p>
  
  <p>But it doesn't work. </p>
  
  <p>Any ideas on a query to do that?</p>