title: Update every row with a value from another column from same table
body: <p>I have the following example of table content</p>
  
  <pre><code>+ Column1 | Column2 | Column3 +
  +---------|---------|---------+
  +   1     | val.txt |         +
  +   2     | test.xls|         +
  +   3     | abc.dwg |         +
  +   4     | y.txt   |         +
  +---------|---------|---------+
  </code></pre>
  
  <p>I want to update <code>Column3</code> with information from <code>Column2</code>, which means I want to (in this case) extract the extension from <code>Column2</code> and put it in <code>Column3</code>.</p>
  
  <p>I want the result to look like</p>
  
  <pre><code>+ Column1 | Column2 | Column3 +
  +---------|---------|---------+
  +   1     | val.txt |   .txt  +
  +   2     | test.xls|   .xls  +
  +   3     | abc.dwg |   .dwg  +
  +   4     | y.txt   |   .txt  +
  +---------|---------|---------+
  </code></pre>
  
  <p>How to do that with an <code>UPDATE</code> statement?</p>
  
  <p>I know how to extract the extension:</p>
  
  <pre><code>SUBSTRING(Column2, LEN(Column2)-3, LEN(Column2)) AS Extension
  </code></pre>