title: Ignore null (empty) value mysql SELECT AVG
body: <p>I have a php table which counts the AVG from mysql table. 
  My problem is that when i have a null value(empty field) on my database table, it's counted like 0 value so the avg result will be wrong.
  For example, i have this on my table:</p>
  
  <pre><code>userID   question1     
  
   1         NULL    
   2          7 
  </code></pre>
  
  <p>When i make a <code>SELECT AVG(question1)from table</code> i get: <code>3,5</code> as result(it takes 0 and 7 as values) and that's wrong because user 1 didn't answer for <code>question1</code> yet, I want to ignore the null value to get AVG result = 7 (taking only 7 for value).
  Is there any way to do this?</p>