方法1:
$this->db->get_where('test','a=1 or (b=1 and c=1)');
方法2:
$this->db->or_where('a=1',NULL,FALSE);
$this->db->or_where('(b=1 and c=1)',NULL,FALSE);
$this->db->get('test');
方法3:
$this->db->where('id=1 or (id=1 and id=1)',NULL,FALSE);
$this->db->get('test');