QueryBuilder inside

I'm not sure if this is the best way but I've been using queryBuilder to make the sub query then a different instance for the outer query.

PHP Code:

$subqueryBuilder = $this->db->table('cs_web_display');
$subqueryBuilder->select('site_id');
...
$subquery = $subqueryBuilder->getCompiledSelect(); 


and then plug $subquery into your outer $builder