我认为您不需要任何SQL查询。

<?php

$args = array(
    \'role\'         => \'author\', // authors only
    \'orderby\'      => \'registered\', // registered date
    \'order\'        => \'DESC\', // last registered goes first
    \'number\'       => 1 // limit to the last one, not required
);

$users = get_users( $args );

$last_user_registered = $users[0]; // the first user from the list

echo $last_user_registered->ID; // print user ID