Database 规范化-重复使用外键

当规范化数据时，在同一个表中重复一个外键是否可以接受

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e、 g.快递公司有一个订单表格和一个客户表格，订单表格将记录从哪个客户取货（客户ID），并且还将有一个列，显示它要交付给哪个客户（也是客户ID）。

这是完全可以接受的；这两个索引将分别维护。这两种关系可以分别使用

select item, name, address from orders inner join customers on orders.sender = customers.id where orders.id = 5;
select name, address from orders inner join customers on orders.recipient = customers.id where orders.id = 5;

或者一起（小心地）

一定要小心级联删除；）

select item, name as sender_name, address as sender_address, recipients.name as as recipient_name, recipients.address as recipient_address from orders inner join customers on orders.sender = customers.id inner join customers as recipients on orders.recipient = recipients.id;