What is the best way to implement Polymorphic Association in SQL Server?

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Another common Name for this model is the Supertype Model, where one has a base set of attributes that can be expanded via joining to another entity. In Oracle books, it is taught both as a logical model and physical implementation. The model without the relations would allow data to grow into invalid state and orphan records I would strongly validate the needs before selecting that model. The top model with the relation stored in the base object would cause nulls, and in a case where fields were mutually exclusive you would always have a null. The bottom diagram where the key is enforced in the child object would eliminate the nulls but also make the dependency a soft depenendency and allow orphans if cascading was not enforced. I think assessing those traits will help you select the model that fits best. I have used all three in the past.

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