title: mysql get average data for full months
body: <p>Given the following sample data:</p>
  
  <p><strong>tblData</strong></p>
  
  <pre><code>Date           Sales
  ----------------------
  2011-12-01       122
  2011-12-02       433
  2011-12-03       213
  ...
  2011-12-31       235
  
  2011-11-01       122
  2011-11-02       433
  2011-11-03       213
  ...
  2011-11-30       235
  
  2011-10-10       122
  2011-10-11       433
  2011-10-12       213
  ...
  2011-10-31       235
  </code></pre>
  
  <p>Notice that October data begins at 10 October, whereas subsequent months have complete data. </p>
  
  <p>I need to get the average monthly sales over all complete months, which in this case would be November and December 2011.</p>
  
  <p>How would I do this?</p>