为什么不能设置NoteContentRelationship.ContentId为外键?您可以轻松地使用关系继承模型，其中表 Content 表示抽象基类，各种表 AnimalContent、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.