stackoverflow.com/a/21435355/2078474

<?php
/**
 * Testing out WP_TAX_Query
 *
 * @link: http://stackoverflow.com/a/21435355/2078474
 */

    $tax_query = array();
    $tax_query[0] = array(
                       'taxonomy'           => 'category',
                       'terms'              =>  array( 65 ),
                       'field'              => 'term_id',
                       'include_children'   => 1
                   );
    $t = new WP_TAX_Query( $tax_query );
    print_r( $t->get_sql( $GLOBALS['wpdb']->posts, 'ID' ) );
?>

Results:

Array
(
    [join] =>  INNER JOIN wp_term_relationships ON (tfl_posts.ID = wp_term_relationships.object_id)
    [where] =>  AND ( wp_term_relationships.term_taxonomy_id IN (15,70,75) )
)