title: SQL aggregate with multiple rows
body: <p>I have the following query</p>
  
  <pre><code>SELECT tagindex, AVG(val) from floatTable
   WHERE tagindex IN(828,856,883,910)
     AND DateAndTime &gt; DATEADD(HH,-1,GETDATE()) 
     AND DateAndTime &lt; DATEADD(HH,-2,GETDATE())
  group by tagindex
  </code></pre>
  
  <p>It returns the following:</p>
  
  <pre><code>828   1
  856   1
  883   1
  910   1
  </code></pre>
  
  <p>How can I return a single result where it is the combined average of all the rows?</p>