Can Foreign Keys Be NULL or Duplicate, and Why Does It Matter?

Foreign key constraints: NULL values ​​and duplicate values

In a relational database, a foreign key is a column in a table that refers to the primary key of another table. Foreign keys are often used to establish relationships between data to ensure data consistency and referential integrity. However, foreign keys can be NULL or duplicate, which is different from the traditional understanding of data integrity.

Can a foreign key be NULL?

Contrary to the belief that foreign keys should not be NULL, foreign keys can actually be NULL. This situation occurs when the value of the foreign key is unknown or unavailable when data is inserted. In this case, the NULL value acts as a placeholder, indicating that the reference is not resolved. Some database management systems allow NULL values ​​in foreign key columns by default, while others require specific constraints to allow them.

Can foreign keys be repeated?

A foreign key can also be repeated, meaning it can reference multiple rows in the parent table. This occurs when the relationship between tables is one-to-many, where one record in the parent table can have multiple corresponding records in the child table. For example, the primary key of the order table is OrderID, which can be referenced by multiple records in the order details table. Each record represents a unique line item in the order.

Why are NULL and duplicate foreign keys allowed?

Understanding why NULL and duplicate foreign keys are allowed is critical to maintaining data integrity. NULL foreign keys allow flexibility in data entry, especially when the reference value may not be immediately available. For example, the sales proposal table may have a foreign key to the salesperson table, but when the proposal is created, the salesperson may not have been assigned yet. In this case, a NULL foreign key allows the proposal record to be inserted without blocking the process.

Duplicate foreign keys represent one-to-many relationships, enabling efficient data partitioning. Consider an employee table with a primary key of EmployeeID and a dependent table HealthInsurance with a foreign key of EmployeeID. Each employee can have multiple health insurance policies, resulting in duplicate foreign key values ​​referencing the same EmployeeID. This makes it possible to easily manage and retrieve insurance policies related to each employee.

By understanding the purpose and implications of allowing NULL and duplicate foreign keys, database designers can create database structures that accommodate complex data scenarios while maintaining data integrity and relationships.

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