title: Add row number to existing column with additional criteria
body: <p>I have a corresponding data structure:</p>
  <pre><code>ItemId ItemDescription
  1      blahblah
  2      blahblah
  3      foobar
  4      foobar
  5      lorem
  6      blahblah
  7      lorem
  </code></pre>
  <p>I wanna append a unique postfix to ItemDescription based on  ItemId order and groupped by ItemDescription value.</p>
  <p>Desired result:</p>
  <pre><code>ItemId ItemDescription
  1      blahblah1
  2      blahblah2
  3      foobar1
  4      foobar2
  5      lorem1
  6      blahblah3
  7      lorem2
  </code></pre>
  <p>I had a successful try with a fixed ItemDescription:</p>
  <pre><code>update item
  set ItemDescription = ItemDescription + cast(row_num as varchar)
  from (select ItemDescription , row_number() over (order by i.ItemId) as row_num 
        from Item i
        where i.ItemDescription = 'lorem') item
  </code></pre>
  <p>But there are thousands of them, so I wanna do it in a single script / couple of them. Looks like I have to use a <code>row_number()</code> function with some kind of nested selects, but no luck so far.</p>