從表中返回的記錄，其對另一臺

Question

在一間DBMS以下三個表的所有記錄的比賽：

Customer(Id, Name, City), 
Product(Id, Name, Price), 
Orders(Cust_Id, Prod_Id, Date) 

什麼是它獲取的客戶查詢（如果有的話），誰下令全部產品？

Answer 1

select c.id 
from customer c 
inner join orders o on o.cust_id = c.id 
inner join product p on p.id = o.prod_id 
group by c.id 
having count(distinct p.id) = (select count(id) from product)