Site map - DB query?

Is there a simple way to create a php script that would run a query against the db and generate a well formatted xml site map file? 
Either daily with a cron job or on demand? 

The other options discussed here and that I found through out the internet, involve a script that spiders your site.  The spider method seems much slower than the DB method, after all the info is sitting right there waiting to be processed.

Are there other advantages to doing one vs the other?

Thanks,
Pat