First, you need to setup relations. After that, you can run your query like this;

$province = Province::with('regioni')->get(array(
    'province.id AS provincia_id',
    'province.name AS provincia',
    ...
));

var_dump($province);

Tnx ChrisRM, but I would like only certain fields

That's what get(array()) does. You can define which columns to retrieve in the same way as you do select(array()).

I need of a "where" clause to filter only some records: How can I convert this query?

         return \DB::table('province')
                  ->join('regioni', 'province'.'.regione_id', '=', 'regioni.id')
                  ->select('province'.'.id', 'province'.'.name AS provincia', 'province'.'.sigla', 'province'.'.slug',
                    'regioni.name AS regione', 'regioni.slug AS regione_slug', 'regioni.id AS regione_id')
                  ->where($relationName.'.slug', $slug)
                  ->get();

I have try this:

         return self::with(array('regione', 'provincia'))->whereHas($relationName, function($q) use ($slug) { 

                  $q->where('slug', $slug);

         })->get();

and works!

I there a best way?