Many a times when we are working with drupal views there comes situations when you need to alter query based on dynamic conditions, at such times drupal hook_views query becomes very handy as you can control the conditions in code. For example consider a senario where you want to add condition in views if a particular SESSION is set

function mymodule_views_query_alter(&$view, &$query) {
  switch($view->name) {
    case 'myview': 
      if (isset(S_SESSION['mysession']) && ($view->current_display == 'page')) {
        $view->query->where[1][conditions][] = array(
          'field' => 'node.language',
          'value' => array($language->language),
          'operator' => 'in'
        );
      }
      break;
   }
}

This will add condition to view query based on the SESSION value.