improve Query Builder to make more complex queries

I suggest query builder should have some options:

1 - easy to make a sub query
today i used...:

$this->db->select('field');
$this->db->from('table')
$this->db->where('field', 'sub select here make in hand ... e.g select field from outher table where outher field= field')

I suggest..

$dbObject = $this->db->select('field')->from('table')->where('');
$this->db->select('field)->from('table')->where('field', $dbObject);

2 - easy to make a union

$dbObject1 = $this->db->select('field')->from('table')->where('');
$dbObject2 = $this->db->select('field')->from('table')->where('');

$this->db->select()->union($dbObject1, $dbObject2);