许多表之一的外键?

问题描述

设置外键约束的常用方法是选择外键将指向哪个表.

The usual way of setting a foreign key constraint is to choose which table the foreign key will point to.

我在 1 个表和一组表之间存在多态关系.

I'm having a polymorphic relation between 1 table and a set of table.

这意味着该表将与集合中的其中一个表有关系.

That means that this table will have a relation with one of those tables in the set.

例如

images: person_id, person_type
subordinates: id, col1, col2...col9
products: id, colA, colB...colZ

在上面的例子中，如果person_type是subordinates"，那么person_id应该是subordinates.id的外键，产品也是如此.

In the above example, if person_type is "subordinates" then person_id should be a foreign key to subordinates.id and the same goes with products.

所以我想知道，是否可以有一个外键指向多个表中的一个，或者您是否必须在分配一个表时专门设置它指向哪个表.

So I wonder, is it possible to have a foreign key to one of many tables, or do you have to specifically set which table it points to when you assign one.

这个问题是针对 MySQL 和 PostgreSQL 的.

This question is for both MySQL and PostgreSQL.

谢谢

推荐答案

不，外键约束总是只引用一个父表.

No, a foreign key constraint always references exactly one parent table.

这个问题经常出现.以下是我过去的一些回答:

This question comes up frequently. Here are some of my past answers to it:

• 为什么多态关联中不能有外键吗?
• 可以为两个可能的表之一设置 MySQL 外键吗?
• 在同一列中引用外键
• 在 StackOverflow 克隆中，Comments 表应该与 Questions and Answers 有什么关系?
• MySQL - 条件外键约束
• 如何处理ERD(表格)设计中的OR"关系?
• MySQL:两个 n:1 关系，但不能同时使用

• Why can you not have a foreign key in a polymorphic association?
• Possible to do a MySQL foreign key to one of two possible tables?
• Referencing foreign keys in the same column
• In a StackOverflow clone, what relationship should a Comments table have to Questions and Answers?
• MySQL - Conditional Foreign Key Constraints
• How to handle an "OR" relationship in an ERD (table) design?
• MySQL: Two n:1 relations, but not both at once

有关多态关联的更多信息，请参阅我的演示文稿 Practical Object-SQL 中的面向模型 或我的书 SQL 反模式:避免数据库编程的陷阱.

For more on Polymorphic Associations, see my presentation Practical Object-Oriented Models in SQL or my book, SQL Antipatterns: Avoiding the Pitfalls of Database Programming.

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