尝试一下

$this->db->set('ati.time_index_default', "CASE WHEN ati.time_index_value = 'PM' THEN acg.group_pm_default ELSE acg.group_am_default END", FALSE);
$this->db->where('s.user_id', 123);
$this->db->update('attendance_time_index ati
join users s on ati.time_index_user_id = s.user_id
join attendance_code_groups acg  on s.user_attendance_code_group = acg.group_id
');