title: Add auto incrementing number based on column
body: <p>I am trying to wrap my head around a problem I hit exporting data from one system to another. </p>
  
  <p>Let's say I have a table like:</p>
  
  <pre><code>id  |  item_num  
  1      1            
  2      1            
  3      2            
  4      3            
  5      3           
  6      3            
  </code></pre>
  
  <p>I need to add a column to the table and update it to contain an incrementing product_num field based on item. This would be the end result given the above table. </p>
  
  <pre><code>id  |  item_num  |  product_num
  1      1            1
  2      1            2
  3      2            1
  4      3            1
  5      3            2
  6      3            3 
  </code></pre>
  
  <p>Any ideas on going about this? </p>
  
  <p>Edit: This is being done in Access 2010 from one system to another (sql server source, custom/unknown ODBC driven destination)</p>