using or_where() on same field

[eluser]Armchair Samurai[/eluser]
Another solution, which doesn't require hacking the CI core. Slightly messy, but gets the job done.

Code:

$start_date = $this->db->escape($start_date);
$sql = "(gm.resign_date >= $start_date OR gm.resign_date IS NULL)";

$this->db->join('groups_members gm', 'gm.member_id = m.user_id');
$this->db->where('gm.join_date <=', $end_date);
$this->db->where($sql, NULL, FALSE);
$this->db->where('gm.group_id', $this->session->userdata('group_id'));
$this->db->order_by('last_name', 'asc');

$query = $this->db->get('members m');