title: MySQL Join Same Table
body: <p>I have the table 'meta_data' with the following fields: </p>
  
  <ul>
  <li>id </li>
  <li>post_id </li>
  <li>meta_key </li>
  <li>meta_value</li>
  </ul>
  
  <p>I'd like to loop through and display a list of EACH post (<code>post_id</code>) that has an entry for <code>meta_key='abc'</code> but <em>not</em> one for <code>meta_key='def'</code></p>
  
  <p>Basically, every post that has a <code>meta_key='abc'</code> entry <em>should</em> have a <code>meta_key='def'</code> entry. I want to generate the list so I can add the missing <code>meta_key='def'</code> entries. </p>