title: How to add a identity column that is unique per grouping
body: <p>Say I have 2 columns</p>
  
  <pre><code>Product     Product_Cat
  -------     -----------
  Cat         0
  Dog         0
  Potatoes    2
  Carrots     2
  Laundry     1
  Bird        0
  </code></pre>
  
  <p>I want to add on a 3rd identity column but I want the numbering to be unique per Product_Cat
  So the output would look like</p>
  
  <pre><code>Product     Product_Cat     Cat_Ident
  -------     -----------     ---------
  Cat         0               1
  Dog         0               2
  Potatoes    2               1
  Carrots     2               2
  Laundry     1               1
  Bird        0               3
  </code></pre>
  
  <p>How do you do this?</p>
  
  <hr>
  
  <p>This of course is not my real data but a simplification of what I want to do. in my live system I have 4585 different values of "Product_Cat" and they range from 1 to 2408 "Products" in the category.</p>