title: Wordpress: Query Posts without allocated taxonomies
body: <p>I would like to filter all post queries by a specific term of a custom taxonomy (specified by an url). Currently I'm using add_filter('request', ...) and 'tax_query', which kind of works now.</p>
  
  <p>So I get all posts of this term, but I would like to get those posts, which do not have ANY terms allocated of this taxonomy.</p>
  
  <p>I was trying something like:</p>
  
  <pre><code>$request['tax_query'] = array(
      'relation' =&gt; 'OR',
      array(
          'taxonomy' =&gt; 'brand',
          'field' =&gt; 'slug',
          'terms' =&gt; array( $term )
      ),
      array(
          'taxonomy' =&gt; 'brand',
          'field' =&gt; 'slug',
          'terms' =&gt; NULL,
          'operator' =&gt; 'IS'
      ),
  );
  </code></pre>
  
  <p>But, since Wordpress doesn't support 'IS' or 'IS NULL' this doesn't work either.</p>
  
  <p>So, any idea how to query for posts of a term AND posts without a term at all?</p>
  
  <p>EDIT: Another solution would be to query all terms, an exclude it then in tax_query. But I would like to save this query.</p>