Handling Complex Foreign Key Relationships in Database Design

Scenario

This example explores a common database design challenge: creating a foreign key relationship where a single table needs to reference primary keys from multiple other tables. The specific scenario involves the deductions table needing to link to both employees_ce and employees_sn tables.

The Problem and its Solution

The question is whether a foreign key in deductions can directly reference primary keys in both employees_ce and employees_sn simultaneously. The answer is: not directly, in a standard relational database. A single foreign key can only reference a single primary key.

The proposed solution leverages a well-structured inheritance model to resolve this.

Inheritance-Based Approach

The tables employees, employees_ce, and employees_sn represent an inheritance hierarchy. employees is the base table, with employees_ce and employees_sn as specialized tables inheriting from it.

Database Structure:

employees (id INT PRIMARY KEY, name VARCHAR(255))
employees_ce (id INT PRIMARY KEY, ce_specific_attribute VARCHAR(255), employee_id INT, FOREIGN KEY (employee_id) REFERENCES employees(id))
employees_sn (id INT PRIMARY KEY, sn_specific_attribute VARCHAR(255), employee_id INT, FOREIGN KEY (employee_id) REFERENCES employees(id))

Solution:

Instead of a direct link to both employees_ce and employees_sn, the deductions table should reference the common parent table, employees:

deductions (id INT PRIMARY KEY, employee_id INT, deduction_amount DECIMAL(10,2), ..., FOREIGN KEY (employee_id) REFERENCES employees(id))

This approach ensures referential integrity and avoids redundancy. The employee_id in deductions links to the employee record in employees, regardless of whether that employee has additional information in employees_ce or employees_sn. This design efficiently manages the relationship and maintains data consistency.