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SELECT customer_id, order_cnt, totalBestellwertUser
FROM (
	SELECT customer_id,
		COUNT(customer_id) 						AS order_cnt,
	    SUM(totalBestellwert) 					AS totalBestellwertUser
   	FROM (SELECT so.customer_id,
          	YEAR(so.created_at) 				AS year_ordered,
            SUM(base_grand_total) 				AS totalBestellwert,
            group_concat(si.sku SEPARATOR ',') 	AS skus
	      FROM `sales_flat_order` 				AS so
	      INNER JOIN `sales_flat_order_item` 	AS si ON si.order_id=so.entity_id
	      WHERE so.customer_id IS NOT NULL
	      GROUP BY entity_id 
    ) sub_query

 	GROUP BY customer_id
 ) main_sub_query