ThinkPHP搜索条件是数组，加一条自定义Sql条件解决办法

// 原来的搜索条件
$where = array(
'user_examinfos.end_at IS NOT NULL',
'examinations.exam_type' => ExaminationsModel::EXAM_TYPE_EXAM,
'user_examinfos.examinationid' => $examinationid,
'user_examinfos.userid' => array('in', $userIds)
);

// 办法一
$where['_string'] = 'user_examinfos.score > user_examinfos.exam_pass_score';

// 办法二
$where[] = 'user_examinfos.score > user_examinfos.exam_pass_score';

$result = M('user_examinfos')
->join('examinations on user_examinfos.examinationid = examinations.id', 'LEFT')
->where($where)
->select();