title: MySQL: return posts if meta_key does NOT exist
body: <p>I have a query that returns a pet post if the meta_key exists and is not '':</p>
  
  <pre><code>-- Pets that are assigned tags
  SELECT p.ID
  FROM FxFfekP0_posts p LEFT JOIN FxFfekP0_postmeta pm
  ON p.ID = pm.post_id
  WHERE p.post_type = 'pets'
  AND pm.meta_key = 'tag'
  AND pm.meta_value != ''
  </code></pre>
  
  <p>But now I need to get a result set that contains posts that either do not contain the meta_key <code>tag</code> at all, or the meta_key <code>tag</code> = '' ...</p>
  
  <p>Help?</p>