Dealing with Foreign Keys to Multiple Tables

You have three tables: regions, countries, and states. Countries and states can belong to regions, forming a hierarchical structure. Now, you want to create a "popular_areas" table with columns "region_id" and "popular_place_id" and establish a foreign key relationship between "popular_place_id" and either countries or states based on a "popular_place_type" column.

This scenario presents a challenge because SQL foreign key constraints typically reference a single target table. However, there is a concept known as Polymorphic Associations in database design which attempts to address this.

Polymorphic Associations: Joining Multiple Tables with a Common Supertype

Polymorphic Associations allow a foreign key to refer to one of several possible target tables. This is achieved by using an additional column to specify the target table being referenced. For instance, in your case, you would need a "popular_place_type" column to determine whether the "popular_place_id" is referencing a country or a state.

However, Polymorphic Associations have limitations in SQL due to constraints. SQL cannot enforce database consistency with polymorphic associations. Instead, it relies on the application or framework implementing these associations to ensure data integrity.

Alternative Solutions for Foreign Key References to Multiple Tables

To maintain referential integrity without Polymorphic Associations, consider these alternatives:

• Creating Separate Tables: Establish tables such as "popular_states" and "popular_countries," each referencing states and countries, respectively. This provides data integrity through SQL constraints but may require multiple queries to retrieve all popular places associated with a user.
• Creating a Parent Table: Create a "places" table that acts as a supertype for states and countries. Both states and countries would have foreign keys referencing "places," ensuring referential integrity.
• Using Two Columns: Instead of a single column referencing either states or countries, use two nullable columns: "state_id" and "country_id." However, you must ensure that only one of these columns is non-NULL and enforce this constraint through application logic or database triggers.

Relational Integrity and the Violation of Normal Forms

It's important to note that Polymorphic Associations violate database normalization principles. First Normal Form (1NF) prohibits mixing different meanings in a single column, which occurs when storing both state and country IDs in a single "popular_place_id" column. Polymorphic Associations also violate Third Normal Form (3NF) because the meaning of the "popular_place_id" column depends on the additional "popular_place_type" column.

In summary, Polymorphic Associations offer a potential solution for foreign key relationships to multiple tables in SQL. However, they introduce data integrity challenges and violate database normalization principles. Consider alternative approaches mentioned above for robust data management.