Table: Product
+--------------+---------+ | Column Name | Type | +--------------+---------+ | product_id | int | | product_name | varchar | | unit_price | int | +--------------+---------+ product_id 是这张表的主键
Table: Sales
+-------------+---------+ | Column Name | Type | +-------------+---------+ | seller_id | int | | product_id | int | | buyer_id | int | | sale_date | date | | quantity | int | | price | int | +------ ------+---------+ 这个表没有主键，它可以有重复的行. product_id 是 Product 表的外键.
编写一个 SQL 查询，查询购买了 S8 手机却没有购买 iPhone 的买家。注意这里 S8 和 iPhone 是 Product 表中的产品。
查询结果格式如下图表示：
Product table:
+------------+--------------+------------+
| product_id | product_name | unit_price |
+------------+--------------+------------+
| 1 | S8 | 1000 |
| 2 | G4 | 800 |
| 3 | iPhone | 1400 |
+------------+--------------+------------+
Sales table:
+-----------+------------+----------+------------+----------+-------+
| seller_id | product_id | buyer_id | sale_date | quantity | price |
+-----------+------------+----------+------------+----------+-------+
| 1 | 1 | 1 | 2019-01-21 | 2 | 2000 |
| 1 | 2 | 2 | 2019-02-17 | 1 | 800 |
| 2 | 1 | 3 | 2019-06-02 | 1 | 800 |
| 3 | 3 | 3 | 2019-05-13 | 2 | 2800 |
+-----------+------------+----------+------------+----------+-------+
Result table:
+-------------+
| buyer_id |
+-------------+
| 1 |
+-------------+
id 为 1 的买家购买了一部 S8，但是却没有购买 iPhone，而 id 为 3 的买家却同时购买了这 2 部手机。
# Write your MySQL query statement below
SELECT buyer_id
FROM
(
SELECT buyer_id
,CASE WHEN p.product_name = 'S8' THEN 1 ELSE 0 END AS s8
,CASE WHEN p.product_name = 'iPhone' THEN 1 ELSE 0 END AS iPhone
FROM Product p
JOIN Sales s
ON p.product_id = s.product_id
) t
GROUP BY buyer_id
HAVING SUM(S8) > 0 AND SUM(iPhone) = 0;