题解 | #返回购买 prod_id 为 BR01 的产品

大量题型传送门：https://github.com/fyupeng/study-notes
使用这种套娃子查询，数据量大的时候性能很差

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'
    )
)​

使用内部连接数据量大的时候性能较高

select cust_email
from OrderItems oi
inner join Orders o
inner join Customers c
on oi.order_num = o.order_num and
o.cust_id = c.cust_id
where prod_id = 'BR01'