OrderItems表和Orders表通过键order_num连接，Orders表和Customers表通过键cust_id连接，可以利用这两个键进行子查询。

SELECT cust_email
FROM Customers
WHERE cust_id IN (
    SELECT cust_id 
    FROM Orders
    WHERE order_num IN
    (
        SELECT order_num
        FROM OrderItems
        WHERE prod_id = 'BR01'
    )
)