title: SQL Where not equal does not return NULL results
body: <pre><code>table 'materials'
  id   sku  content
  10   IT2   Iron
  11   IT3   Steel
  12   IT4   Steel
  13   IT5   NULL
  14   IT6   Iron
  15   IT7   Glass
  
  select id, sku, content from materials where content !='Iron';
  </code></pre>
  
  <p>Returns Result:</p>
  
  <pre><code>id   sku   content
  11   IT3   Steel
  12   IT4   Steel
  15   IT7   Glass
  </code></pre>
  
  <p>Why is id #13 with the NULL value not returned in the result set?  Using MYSQL.</p>