title: Subtract same number for all SQL rows in table
body: <p>In SQL Server 2008 I have a user table like the following: </p>
  
  <pre><code>Userid
  Username 
  UserBirthYear(int)
  </code></pre>
  
  <p>When a user registers to the page, she choses her birthyear. Former programmer (of course!) let the user chose the correct year, but the value which was inserted into the db was DateTime.Now...</p>
  
  <p>Now I need to subtract and save to the database 15 years from each user in coloumn UserBirthYear. There is 500k users. How is this best achieved?</p>
  
  <p>Thanks in advance.</p>