sql - 如何为以下查询编写关系代数表达式？

Question

如何为以下查询编写关系代数表达式？

select Customer_ID
from tbl_Reservation
where Customer_ID not in (select Customer_ID from tbl_Bill)

Answer 1

像这样的东西：

excludedCustomers = π(Customer_ID)(tbl_Bill)
customerReservations= π(Customer_ID)(tbl_Reservation)
result = customerReservations- excludedCustomers

您只需要获取您想要排除的客户的预测（全部来自 tbl_Bill），对 tbl_Reservation 执行相同操作，然后从两个预测中减去就是您的答案。