Can Foreign Key Constraints Reference Multiple Tables?

Polymorphic Foreign Key Constraints: Referencing Multiple Tables

In the realm of database design, foreign key constraints establish relationships between tables, ensuring the integrity of data by connecting records across tables. Traditionally, foreign keys point to a single specific table. However, the question arises: is it possible to create a foreign key that references one of multiple tables?

To understand this concept, consider a polymorphic relationship between a table and a set of tables. In such a scenario, a table can be related to any one of a group of tables. For instance, consider three tables:

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

In this example, if the person_type column in the images table contains "subordinates", then person_id should be a foreign key referencing subordinates.id. Similarly, if person_type is "products", then person_id should reference products.id.

The Answer: Not Possible for Single Foreign Key

After careful examination, it becomes clear that it is not possible to have a single foreign key constraint that references multiple tables. A foreign key constraint always targets precisely one parent table. Therefore, if a field requires referencing multiple tables based on some condition, alternative solutions must be explored.

Additional Resources for Polymorphism

For a deeper understanding of polymorphic associations, consider exploring the following resources:

• Presentation: Practical Object-Oriented Models in SQL
• Book: SQL Antipatterns, Volume 1: Avoiding the Pitfalls of Database Programming

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