How to sort by SKU (sku) in Opencart?

Question

MySQL

In the database, the SKU field has the varchar type, so sorting by article does not work correctly:
1,10,11,2,20,21
How to sort the article as a number?
1,2,10,11,20,21
Which query in /catalog/model/catalog/product.php should be changed?

Answer 1

I figured it out for those who are interested:
On 1.5.3.1
File /catalog/model/catalog/product.php
In this condition:
Add:

elseif ($data['sort'] == 'p.sku') {
         $sql .= " ORDER BY CAST(" . $data['sort'] . " as unsigned)";
        }

Answer 2

instead of ORDER BY xxx
ORDER BY CAST(xxx AS SIGNED)
or ORDER BY xxx+0