title: Multiplying two columns from different data frames in spark
body: <p>I have two dataframes representing the following csv data:</p>
  
  <pre><code>Store   Date    Weekly_Sales
  1   05/02/2010  249
  2   12/02/2010  455
  3   19/02/2010  415
  4   26/02/2010  194
  
  Store   Date    Weekly_Sales
  5   05/02/2010  400
  6   12/02/2010  460
  7   19/02/2010  477
  8   26/02/2010  345
  </code></pre>
  
  <p>What i'm attempting to do is for each date, read the associated weekly sales for it in both dataframes and find the average of the two numbers. I'm not sure how to accomplish this.</p>