title: How to get percentage of rows which are not NULL in a specific column?
body: <p>I have a database setup like this:</p>
  
  <p><strong>Table called <code>reviews</code></strong></p>
  
  <pre><code>+ -------- +
  |  review  |
  + -------- +
  |   awda   |
  |   ggagw  |
  |   okok   |
  |   ligjr  |
  |   kkfm   |
  |   seff   |
  |   oawr   |
  |   abke   |
  |  (null)  |
  |  (null)  |
  |  (null)  |
  |  (null)  |
  |  (null)  |
  |  (null)  |
  |  (null)  |
  + -------- +
  </code></pre>
  
  <p>How do I get the <strong>percentage of how many rows there are, which are NOT NULL</strong>?</p>
  
  <p>A basic "formula" of what I want:</p>
  
  <p>percentage = 100 * ( (Sum of rows where <code>review</code> is not null) / (Amount of rows) )</p>
  
  <p>For the example above, this would be:
  percentage = 100 * ( ( 8 ) / ( 15) )
             = 53.33333333</p>
  
  <p><em>How can I achieve that by using only one MySQL query?</em></p>