解法1
select cust_email from OrderItems as os left join Orders o on os.order_num = o.order_num left join Customers c on o.cust_id = c.cust_id where prod_id = 'BR01'解法2
SELECT c.cust_email cust_email FROM OrderItems oi INNER JOIN Orders o ON (oi.prod_id = 'BR01') AND (oi.order_num = o.order_num) INNER JOIN Customers c ON o.cust_id = c.cust_id;解法3