title: Average ratio between two columns
body: <p>I've been collecting tweets, and running some queries on them.
  In one of the queries I'm trying to find all users that have an an average follower to friend ratio of at least two (i.e. twice as many followers as friends). Finding the average is important because I have collected the tweets of some users multiple times, at different times. Their friends or followers can change, so they can come up with multiple ratios. </p>
  
  <p>I can do it without averages, resulting in multiple results for some users</p>
  
  <pre><code>SELECT screenname,followers,friends,usertweets, followers/friends AS FFRatio 
  FROM tweets
  WHERE friends &gt;0 AND followers/friends&gt;2
  </code></pre>
  
  <p>When I try to use AVG, I get errors, and no results.</p>
  
  <pre><code>SELECT screenname,followers,friends,usertweets, AVG(followers/friends) AS FFRatio 
  FROM tweets
  WHERE friends &gt;0 AND AVG(followers/friends)&gt;2
  </code></pre>
  
  <p>Can you tell me what is wrong, please?</p>