db_select with OR condition

db_select_with_or_condition.php

<?php
$query = db_select('node', 'n')
  ->fields('n', array('nid'))
  ->condition('type', '[node_type]');

$query->leftJoin('field_data_[field_name_1]', '[alias_1]', '[alias_1].entity_id = n.nid');
$query->leftJoin('field_data_[field_name_2]', '[alias_2]', '[alias_2].entity_id = n.nid');
$db_or = db_or()
  ->condition('[alias_1].[field_name_1]_value', '0')
  ->condition('[alias_2].[field_name_2]_value', NULL);

$query->condition($db_or);
$result = $query->execute()->fetchAll();

foreach ($result as $row) {
  print $row->nid . "\n";
}