title: SQL "select where not in subquery" returns no results
body: <p><em>Disclaimer: I have figured out the problem (I think), but I wanted to add this issue to Stack Overflow since I couldn't (easily) find it anywhere.  Also, someone might have a better answer than I do.</em></p>
  
  <p>I have a database where one table "Common" is referenced by several other tables.  I wanted to see what records in the Common table were orphaned (i.e., had no references from any of the other tables).</p>
  
  <p>I ran this query:</p>
  
  <pre><code>select *
  from Common
  where common_id not in (select common_id from Table1)
  and common_id not in (select common_id from Table2)
  </code></pre>
  
  <p>I know that there are orphaned records, but no records were returned.  Why not?</p>
  
  <p>(This is SQL Server, if it matters.)</p>