我可以使用PHP / mySQL从Wordpress中查询其他数据库吗？

I have a site that is a mix of Wordpress and other PHP pages. Can I query the other database and have the results show within Wordpress?

Yes you can since Wordpress is written in PHP, and PHP allows you to use whatever database you want.

Late to the party, but if you're willing to add stuff to the wp-includes/wp-db.php file you can query those other databases using the $wpdb object.

Specifically, you need to add a var:

/**
     * WordPress Term Relationships table
     *
     * @since 2.3.0
     * @access public
     * @var string
     */
     var $term_relationships;

and add your database name to this:

var $tables = array('users', 'usermeta', 'posts', 'categories', 'post2cat', 'comments', 'links', 'link2cat', 'options','postmeta', 'terms', 'term_taxonomy', 'term_relationships');