Call to undefined function db_query()

Drupal provides the function db_query() to interact with database.

This function is defined in includes/database.inc file.

You have to include database.inc in your file to get db_query() working.

<?php include(”includes/database.inc”); ?>

However if you are trying to access drupal database from a non-drupal .php file then all you need to do is to include following lines at the top of your file.

require_once ‘./includes/bootstrap.inc’;
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);