mysql表中的多个外键到同一主键

I have a table user with userID as the primary key. I have another table called Friends. In the Friends table, I have two Users as friends represented by the columns UserID and FrndID where both UserID and FrndID should be a userID in table user.

我有一个用户ID为主键的表用户。我有另一张名为Friends的桌子。在Friends表中，我有两个User作为朋友，由UserID和FrndID列表示，其中UserID和FrndID都应该是表user中的userID。

I want to enforce data integrity. Could I use something like this?

我想强制执行数据完整性。我能用这样的东西吗？

ADD CONSTRAINT `ufd_users_fk` FOREIGN KEY (`userId`, `friendId`)
REFERENCES `users` (`userId`, `userId`) ON DELETE CASCADE ON UPDATE CASCADE;

I want to know is REFERENCESusers(userId,userId) referencing a column multiple times correctly? The reason I am not creating 2 separate constraints, is that both users must exist in table user.

我想知道是REFERENCESusers（userId，userId）多次正确引用列？我没有创建2个单独约束的原因是两个用户必须存在于表用户中。

1 个解决方案

#1

No, you should create two foreign keys:

不，你应该创建两个外键：

ADD CONSTRAINT `ufd_users_fk` FOREIGN KEY (`userId`) 
  REFERENCES `users` (`userId`) 
  ON DELETE CASCADE ON UPDATE CASCADE,
ADD CONSTRAINT `ufd_users_fk` FOREIGN KEY (`friendId`) 
  REFERENCES `users` (`userId`) 
  ON DELETE CASCADE ON UPDATE CASCADE;