title: MySQL "NOT IN" query not working
body: <p>I have a table with three columns: <code>taxon_id</code>, <code>scientific_name_element_id</code>, and <code>parent_id</code>. I want to find the elements that are children and not parents, so the termini of the structure. </p>
  
  <p>I found some sources that suggested that I use</p>
  
  <pre><code>select taxon_id 
         from  taxon_name_element
         where taxon_id not in 
                            (select parent_id from taxon_name_element)
  </code></pre>
  
  <p>But this does not work, I get an empty set when I can actually browse the entries and see that there is, for example, a <code>taxon_id=1</code>, and NO <code>parent_id=1</code></p>
  
  <p>Conversely when I see what taxon_id's are in parent_id's I get a nonempty result set</p>
  
  <p><strong>What am I doing wrong? How can I fix this?</strong></p>