2个外键引用同一张表

Question

我可以在同一个表中有两个外键，这些外键引用另一个名为profil（相同的表）的表吗？

我的表是MailSent，它包含：主键（Id），日期，foreignkey1（profil_sender），foreignkey2（profil_receiver）

Answer 1

我们可以有两个外键引用同一个表，正如“@zhou yun”答案中提到的，为了实现这一点，表将如下所示：

create table MailSent(

  Id int primary key,

  profil_sender_id int,

  profil_receiver_id int,

  FOREIGN KEY (profil_sender_id) REFERENCES profil(id),

  FOREIGN KEY (profil_receiver_id) REFERENCES profil(id)

)

要从这两个表中进行选择并使用两个外键连接这两个表，请求将如下所示：

SELECT ms.*, ps.first_name as name_sender,pr.first_name as name_reciver  
FROM MailSent ms 
LEFT JOIN profil ps 
ON ms.profil_sender_id= ps.id
LEFT JOIN profil pr 
ON ms.profil_receiver_id= pr.id

Answer 2

将外键添加(profil_sender_id, profil_receiver_id)到现有表中(MailSent)，请按照下列步骤操作：

ALTER TABLE MailSent ADD CONSTRAINT fk_profile_sender_id FOREIGN KEY (profil_sender_id) REFERENCES TABLE-NAME(id);

ALTER TABLE MailSent ADD CONSTRAINT fk_profil_receiver_id FOREIGN KEY (profil_receiver_id) REFERENCES TABLE-NAME(id);

Answer 3

如果你想在创建表时添加外键，可以这样做：

create table MailSent(

  Id int primary key,

  date datetime,

  profil_sender int,

  profil_receiver int,

  CONSTRAINT fk_sender FOREIGN KEY (profil_sender) REFERENCES profil(id),

  CONSTRAINT fk_receiver FOREIGN KEY (profil_receiver) REFERENCES profil(id)

)