select_sum in codeigniter with join code example

Example 1: codeigniter where order by

$this->db->select('ae_users.employee_id, ae_users.emp_name, ae_users.emp_name2, ae_users.emp_name3');
		$this->db->from('ae_users');                
		$this->db->where_in('ae_users.employee_id',$employee_ids);
                $this->db->_protect_identifiers = FALSE; // stop CI adding backticks
                $order = sprintf('FIELD(ae_users.employee_id, %s)', implode(', ', $employee_ids));
                $this->db->order_by($order);
                $this->db->_protect_identifiers = TRUE; 
		$query =$this->db->get();
               // echo '<pre>'; print_r($this->db->last_query());exit;
		if ($query->num_rows()) {
			return $query->result_array();
		} else {
			return 0;
		}

Example 2: codeigniter select for update

$table = "my_table";
$id = 1;
$update = ["status"=>"working"];
//Edit just above /\ if you don't need extra "where" clause
$query = $this->db->select()
            ->from($table)
            ->where('id', $id)
            ->get_compiled_select();
$data = $this->db->query("$query FOR UPDATE")->row_array();
$this->db->where('id', $data['id'])->update($table,$update);