Example how to use db_update, db_select. Case: Update comment possibility including revisions.

drupal.php

<?php

  db_update('node') // Table name no longer needs {}
    ->fields(array(
      'comment' => 1,
    ))
    ->condition('type', 'feature')
    ->execute();
  $subquery = db_select('node')
    ->addField('node', 'nid')
    ->condition('node.type', 'feature', '=');
  db_update('node_revision')
    ->fields(array('comment' => '2',))
    ->condition ('node_revision.nid', $subquery, 'IN')
    ->execute();