title: Multiple clauses in one check constraint?
body: <p>I'm trying to get multiple clauses in one check constraint, separated by an <code>OR</code>:</p>
  
  <pre><code>check (stop&gt;start or stop = NULL)
  </code></pre>
  
  <p>where start and stop are timestamp fields. So, <code>stop</code> can be left empty, but if it's specified, it has to be later than <code>start</code>. </p>
  
  <p>Am I getting this entirely wrong? I haven't found anything on timestamp comparisons or if <code>OR</code> is allowed in check constraints.</p>
  
  <p>This is in postgres 8.4.8.</p>