Design Meets Code

// search all taxonomies, based on: http://projects.jesseheap.com/all-projects/wordpress-plugin-tag-search-in-wordpress-23

function atom_search_where($where){
  global $wpdb;
  if (is_search())
    $where .= "OR (t.name LIKE '%".get_search_query()."%' AND {$wpdb->posts}.post_status = 'publish')";
  return $where;
}

function atom_search_join($join){
  global $wpdb;
  if (is_search())
    $join .= "LEFT JOIN {$wpdb->term_relationships} tr ON {$wpdb->posts}.ID = tr.object_id INNER JOIN {$wpdb->term_taxonomy} tt ON tt.term_taxonomy_id=tr.term_taxonomy_id INNER JOIN {$wpdb->terms} t ON t.term_id = tt.term_id";
  return $join;
}

function atom_search_groupby($groupby){
  global $wpdb;

  // we need to group on post ID
  $groupby_id = "{$wpdb->posts}.ID";
  if(!is_search() || strpos($groupby, $groupby_id) !== false) return $groupby;

  // groupby was empty, use ours
  if(!strlen(trim($groupby))) return $groupby_id;

  // wasn't empty, append ours
  return $groupby.", ".$groupby_id;
}

add_filter('posts_where','atom_search_where');
add_filter('posts_join', 'atom_search_join');
add_filter('posts_groupby', 'atom_search_groupby');

Quelle: WordPress Stackexchange