mysql 一个表中的多个外键指向同一个主键
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mysql Multiple Foreign Keys in a Table to the Same Primary Key
提问by Bill Karwin
I have a table userwith userIDas the primary key. I have another table called Friends. In the Friendstable, I have two Users as friends represented by the columns UserIDand FrndIDwhere both UserIDand FrndIDshould be a userIDin table user.
我有一个表user与userID作为主键。我还有一张桌子叫Friends. 在Friends表中，我有两个用户的朋友表示由列UserID和FrndID其中两个UserID和FrndID应该是一个userID表user。
I want to enforce data integrity. Could I use something like this?
我想强制执行数据完整性。我可以使用这样的东西吗？
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REFERENCES用户(userId ,userId)是否正确多次引用一列？我没有创建 2 个单独的约束的原因是两个用户都必须存在于 table 中user。
回答by Bill Karwin
No, you should create two foreign keys:
不，您应该创建两个外键：
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;
