How to get data of joined table using query() method

Hello everyone.
I have an issue with getting data of the joined table if I use query().
I did not get the data of product table, how can I solve this using query() without using Active record?

Here is my db table structure

PHP Code:

category table
+--------------------------+
| cId | added  | category  |       
+-----+--------+-----------+
|  1  | 1.2.20 |  PC       |
|  2  | 1.7.20 | electron  |
+-----+--------+-----------+ 


PHP Code:

product table
+--------------------------+
| id  |  cId   |  cost     |       
+-----+--------+-----------+
|  1  |   1    |  3000     |
|  1  |   2    |  9000     |
+-----+--------+-----------+ 


MY MODEL

PHP Code:

protected $table = 'category';

public function showProduct(){
    $sql = "SELECT category.*, COALESCE(SUM(product.cost),0) as price 
            FROM category JOIN product ON product.cId = category.cId
            GROUP BY category.cId";
    $this->db->query($sql);
    return $this;
} 


MY CONTROLLER

PHP Code:

public function index()
{
    $result = $this->model->showProduct();
    echo "<pre>";
    print_r($result->asObject()->paginate(1));    
        //pagination
    $pager = $this->model->showProduct()->pager->links();
} 


Result I get

PHP Code:

Array(
    [0] => stdClass Object
        (
            [cId] => 1
            [added] => 1.2.20
            [category] => PC
        )
    [1] => stdClass Object
        (
            [cId] => 2
            [added] => 1.7.20
            [category] => electron
        ),
)