title: choose minimum date from a record out of multiple columns
body: <p>Hi I have the following table struct:</p>
  
  <pre><code>Person    Date1             Date2............Daten
  ------    -----             -----            -----
  1         2001-01-01        2002-01-01
  2         2003-01-01        2000-01-01
  </code></pre>
  
  <p>and i want to choose the minimum Date between Date1 and Date(n)   (20 dates in my case). So for example it would choose Date1 for Person1 and Date2 for Person2.</p>
  
  <p>obviously i can just use min(Date) if I only have 1 date columns, but I can't get my logic right in this case.</p>
  
  <p>Thanks very much.</p>