购买 prod_id 为 BR01 的产品的所有顾客电子邮件

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"
            )
    )

子查询 套娃操作 一定明确每一层相关联的字段和需要查询的字段 cust_id 是关联字段就要在子查询的查询关联字段