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Prøv dette

SELECT a.customer, count(a.sale), max_sale
FROM sales a
INNER JOIN (SELECT customer, sale max_sale 
            from sales x where dates = (select max(dates) 
                                        from sales y 
                                        where x.customer = y.customer
                                        and y.sale > 0
                                       )

           )b
ON a.customer = b.customer
GROUP BY a.customer, max_sale;

SQL DEMO