想将AND 换成 OR,但不知如何写

输出的SQL语句：
string(157) "SELECT * FROM `zhenbao_page` WHERE `parent_id` IN ('12') AND ( ( `department_id` IN ('1') AND `department_id` IN ('') ) ) ORDER BY sort,id desc LIMIT 0,4 "

代码：
$where2['parent_id']=array('in',$id);
$where2['department_id']=array([['in',session('did')],['in','']]);

$list = M('page')->fetchSql()->where($where2)->order('sort,id desc')->select();