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

这样的事情：

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

您只需要获取要排除的客户的投影（全部来自tbl_Bill），对tbl_Reservation执行相同的操作，并从两个投影中减去这些是您的答案。