gistfile1.php

<?php

public function subscribeEvents()
{
	Backend::$events->addEvent('shop:onExtendCategoryModel', $this, 'extend_category_model');
}


public function extend_category_model($category, $context)
{
	$total_sales = "SELECT count(totals_shop_order_items.id)*totals_shop_order_items.quantity
		FROM shop_products_categories as totals_shop_products_categories
			LEFT JOIN shop_categories totals_shop_categories
				ON totals_shop_products_categories.shop_category_id = totals_shop_categories.id
			LEFT JOIN shop_products totals_shop_products  
				ON totals_shop_products_categories.shop_product_id = totals_shop_products.id
			LEFT JOIN totals_shop_order_items totals_shop_order_items
				ON totals_shop_products.id = as totals_shop_order_items.shop_product_id
			LEFT JOIN shop_orders totals_shop_orders
				ON totals_shop_order_items.shop_order_id = totals_shop_orders.id
		WHERE totals_shop_orders.payment_processed IS NOT NULL
		AND totals_shop_categories.id = shop_categories.id
	";
	
	$category->define_column('total_sales', 'Total sales');
	$category->calculated_columns['total_sales'] = array(
		'sql' => $total_sales,
		'type' => db_number
	);
}