方法一:

SELECT

    cust_email

FROM

    Customers AS C

    LEFT JOIN Orders AS O ON C.cust_id = O.cust_id

    LEFT JOIN OrderItems AS ot ON ot.order_num = O.order_num

WHERE

    ot.prod_id = 'BR01';

方法二:

SELECT

    cust_email

FROM

    Customers

WHERE

    cust_id in (

        SELECT

            cust_id

        FROM

            Orders

        WHERE

            order_num IN (

                SELECT

                    order_num

                FROM

                    OrderItems AS ot

                WHERE

                    ot.prod_id = 'BR01'

            )

    )