title: Counting Unique records by Month in Tableau over a year
body: <p>I have a list of product codes and their sale dates. I want to only count the first ever sale of that product, then SUM the first sales by month of sale. 
  So if Product A was sold in Feb, Mar and Nov, it only gets counted in Feb.</p>
  
  <p>Here is a badly copied table</p>
  
  <pre><code>Product Code    Sales Date      WHAT I WANT
  -------------------------------------------
  A1234          01/02/2014       COUNT THIS
  B3333          03/02/2014       COUNT THIS
  C5555          06/03/2014       COUNT THIS
  A1234          09/03/2014       DON’T COUNT
  F567           15/04/2014       COUNT THIS
  P000           25/05/2014       COUNT THIS
  B3333          01/01/2015       DON’T COUNT
  A1234          06/01/2015       DON’T COUNT
  P000           19/01/2015       DON’T COUNT
  B3333          26/01/2015       DON’T COUNT
  K5678          20/02/2015       COUNT THIS
  </code></pre>
  
  <p>I want my end result to be a graph with months down the side and then a count of first times a product is sold. </p>
  
  <p>Does this make sense? I tried using MIN, but it makes everything AGG, which stops you comparing a date with a date. </p>