SQL：一个外键在几个表之一中引用主键 [英] SQL : one foreign key references primary key in one of several tables

问题描述

我正在开发一个将用作其他应用程序的可扩展框架的应用程序。

I am working on an application that will be used as an extensible framework for other applications.

其中一个基本类称为Node，Nodes有Content。 SQL表如下所示：

One of the fundamental classes is called Node, and Nodes have Content. The SQL tables look like this:

TABLE节点（NodeId int，.... etc）

TABLE Node ( NodeId int, .... etc )

NodeContentRelationship（NodeId int，ContentType string，ContentId int）

TABLE NodeContentRelationship ( NodeId int, ContentType string, ContentId int)

这将由开发者扩展应用程序来创建自己的内容类型。

It will be up to the developers extending the application to create their own content types.

显然，从关系数据库的角度来看这是不好的，因为不可能向NodeContentRelationship.ContentId添加外键关系，即使它是外键

Clearly this is bad from a relationship-database point of view as it is not possible to add a foreign key relationship to NodeContentRelationship.ContentId, even though it is a foreign key column.

但是，解决方案非常简单，功能强大，所以我不愿意更改。

However, the solution is quite simple and powerful so I am reluctant to change it.

推荐答案

为什么不能设置 NoteContentRelationship.ContentId 作为外键？您可以很容易地使用一个表格 Content 表示抽象基类的表格和各种表格 AnimalContent code> CarContent 等表示派生类。

Why is it impossible to set NoteContentRelationship.ContentId as a foreign key? You can easily use a relational inheritance model with a table Content representing an abstract base class, and various tables AnimalContent, CarContent, etc. representing derived classes.

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