如何使用$ this-> db-> select_sum和$ this-> db-> group_by（$ groupBy）方法获得求和

I wanted to get summation of a column's value Grouping by another column using Codegniter framework. on model which name is action, i have used this method:

public function read_sum_groupby($table, $column,$groupBy,$where=array()){
    $this->db->group_by($groupBy); 
    $this->db->where($where);
    $this->db->select_sum($column);
    $result = $this->db->get($table);

    return $result->result();
}

and then i called that method on view like this

$installment_discount = $this->action->read_sum_groupby("installment","discount","installment_id",array("loan_id"=>$loan->id));

and then i printed the output like this

<pre>
 <?php print_r($installment_discount); ?>    
</pre>

Result was

you can see the last value is 800 but my expected was 200.

my table was

The installment_id of 11,12,13,14 rows are all 7759702. So if you group by installment_id then select_sum(discount), the sql will be like this:

SELECT sum(discount) FROM installment WHERE loan_id=1 GROUP BY installment_id;

the discount of 11,12,13,14 rows will sum up, so you get 800.

if you want to get 200, the sql should be this:

SELECT sum(distinct(discount)) FROM installment WHERE loan_id=1 GROUP BY installment_id;

your code can be written like this:

$this->db->select_sum("distinct($column)", $column);