SELECT  Sales.SaleID,
        Sales.StaffID,
        Sales.CustomerID,
        Sales.TIMESTAMP,
        Sales.Refunded,
        Sales.PaymentType,
        Staff.Forename AS staffForename,
        Staff.Surname AS staffSurname,
        COALESCE(a.itemList, '') itemList,
        COALESCE(a.sumOfCost, 0) sumOfCost,
        COALESCE(a.sumOfPrice, 0) sumOfPrice,
        COALESCE(a.sumOfCost, 0) - COALESCE(a.sumOfPrice, 0) AS sumOfProfit
FROM    Sales
        INNER JOIN Staff
            ON Sales.StaffID = Staff.StaffID
        LEFT JOIN
        (
            SELECT  SaleID,
                    GROUP_CONCAT(Quantity, ' x ', Name) itemList,
                    SUM(Cost*Quantity) sumOfCost,
                    SUM(Price*Quantity) sumOfPrice
            FROM SaleItems
            GROUP BY SaleID
        ) a ON a.SaleID = Sales.SaleID
WHERE   Sales.Deleted = '0'
ORDER   BY TIMESTAMP DESC
LIMIT   0, 15