Using aggregate function SUM() in CodeIgniter

Model

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
class transaction extends CI_Model{

    public function  mytransaction(){
        $this->load->database();
        $query = $this->db->get('transaction');
        return $query->result();
    }

    public function mySum(){
        $this->db->select("SUM(total) AS MySum");
        $this->db->from("transaction");
        return $query1->row();
    }
}

Controller

class Invoice extends CI_Controller{
    public function index(){
        $this->load->Model('transaction');
        $data1['query'] =  $this->transaction->mytransaction();
        $this->load->view('transactionview', $data1);
    }
}

View

  <table width="600px" border="1px" style="margin:40px 300px;">
        <tr>
            <td>Sno.</td>
            <td>Product Code</td>
            <td>Name</td>
            <td>Quantity</td>
            <td>Rate</td>
            <td>Total</td>
        </tr>


    <?php foreach($query as $row){?>
    <tr>
        <td><?php echo $row->transaction_id;?></td>
        <td><?php echo $row->code;?></td>
        <td><?php echo $row->name;?></td>
        <td><?php echo $row->quantity;?></td>
        <td><?php echo $row->rate;?></td>
        <td><?php echo $row->total;?></td>
    </tr>

    <?php }?>

    <tr>
        <td colspan=4>Subtotal</td>
        <td colspan=2>?</td>
    </tr>

    <tr>
        <td colspan=4>Grand Total</td>
        <td colspan=2>?</td>
    </tr>
    </table>

I am trying to use aggregate function with Codeigniter. For this I want to write a query which sums all total column of the table so that I can echo it to the Grand total field of the table.For this how can I pass the sum value from my model to view so that I can echo sum value in my table?

model function:-

public function mySum(){
        $this->db->select("SUM(total) AS MySum");
        $this->db->from("transaction");
        $query1 = $this->db->get();
        if($query1->num_rows() > 0)
        { 
         $res = $query1->row_array();
         return $res['MySum'];
        }
       return 0.00;
    }

call your model function in your controller:-

 // set grand total here
    $data1['g_total'] =  $this->transaction->mySum();

get in view as:-

    <tr>
        <td colspan=4>Grand Total</td>
        <td colspan=2><?php echo $g_total;?></td>
    </tr>