题解 | 返回购买 prod_id 为 BR01 的产品的所有顾客的电子邮件（一）

两种做法

# 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'
#     )
# );

SELECT cust_email
FROM Customers C
WHERE EXISTS (
    SELECT *
    FROM Orders O
    WHERE C.cust_id = O.cust_id 
    AND EXISTS (
        SELECT 1
        FROM OrderItems OI 
        WHERE OI.prod_id = 'BR01' 
        AND OI.order_num = O.order_num
    )
);