title: where not exists in second table
body: <p>Need to find records in subinv that do NOT have a listing in subinv table with substatus 1
  when there is a listing in inv with status 1.</p>
  
  <p>There is only one record per part number in <code>inv</code>, but can be several records in <code>subinv</code> with one of several status numbers.  My statement returns no rows, shows no errors, but there are records that qualify.</p>
  
  <pre><code>SELECT m.partnum        
    FROM inv m, 
         subinv s     
   WHERE m.status = '1'
     AND not exists (SELECT s.partnum     
                       FROM subinv s1
                      WHERE s1.substatus = '1')       
  </code></pre>