大家好，我正在尝试将数据插入数据库。一些值包含相同的ID。我想在插入过程中求和，而不要添加新行：
if ($q->num_rows() >0)
{
$this->db->where('student_id',$id);
$this->db->set('mark_obtained', ' mark_obtained+ '. $obtained_marks['mark_obtained'], FALSE);
$this->db->update('position' , array('mark_obtained' => $obtained_marks , 'student_id' =>$row['student_id'] , 'exam_id'=> $row['exam_id'],'subject_id' =>$row['subject_id'],'section_id'=>$row['section_id'], 'class_id'=>$row['class_id']));
}
else {
$this->db->insert('position' , array('mark_obtained' => $obtained_marks , 'student_id' =>$row['student_id'] , 'exam_id'=> $row['exam_id'],'subject_id' =>$row['subject_id'],'section_id'=>$row['section_id'], 'class_id'=>$row['class_id']));
}