If you want discount sorting in product list page(category page)

1> First need to create discount attribute for products.

2> After creation that copy  app/code/core/Mage/Catalog/Model/Resource/Product/collection.php in local folder app/code/local/Mage/Catalog/Model/Resource/Product/collection.php

open the file find below bit of code around 1572

     if ($attribute == 'price' && $storeId != 0) {
            $this->addPriceData();
            $this->getSelect()->order("price_index.min_price {$dir}");

            return $this;
        }
after that Add below code

if ($attribute == 'discount') {

            //determine the discount percent value

            $this->addExpressionAttributeToSelect('discount', '(({{price}} - special_price) / {{price}})', array('price'));

            //sort by the discount percent value

            $sortDir = $dir;

            //in case you need to reverse the order so when you sort ascending you will see the most discounts first do this:

            //if not then remove the if-else statement below.

            if ($sortDir == self::SORT_ORDER_ASC) {

                $sortDir = self::SORT_ORDER_DESC;

            }

            else {

                $sortDir = self::SORT_ORDER_ASC;

            }

            $this->getSelect()->order(array('discount '.$sortDir));

            return $this;

        }