title: SQL Server NULL constraint
body: <p>Is is possible in SQL Server 2008 to create such a constraint that would restrict two columns to have NULL values at the same time? So that</p>
  
  <pre><code>Column1 Column2
  NULL    NULL   -- not allowed
  1       NULL   -- allowed
  NULL    2      -- allowed
  2       3      -- allowed
  </code></pre>