Can Foreign Keys Be NULL or Allow Duplicates?

Can foreign keys be empty or repeated?

A foreign key is a column in a table that references the primary key of another table and ensures data integrity by establishing a parent-child relationship. However, foreign key behavior with respect to null values ​​and uniqueness may vary depending on specific database design and constraints.

Can foreign keys be empty?

Yes, foreign keys can be null. When a child record is created, the record referenced in the parent table may not exist, which is usually allowed. For example, the "Sales Orders" table might have a foreign key that references the "Customers" table. Initially, an order may be placed without being assigned a specific customer. Therefore, the foreign key pointing to the customers table can be null until the customer is selected later.

Can foreign keys be repeated?

Yes, foreign keys can be repeated if the relationship between the tables allows it. This is called a many-to-many relationship, where multiple rows in the child table can reference the same row in the parent table. Consider a situation where a "Products" table has a foreign key referencing a "Category" table. A product can belong to multiple categories, so a product can have multiple rows with the same category foreign key value.

The impact of null values ​​and duplication

Allowing null values ​​in foreign keys provides flexibility, but may also introduce data integrity issues. If null values ​​are not handled properly, data operations such as updates or deletes can result in orphaned records (child records without a valid parent).

Duplicate foreign keys, on the other hand, indicate the possibility of data redundancy and inconsistency. If necessary, it is important to enforce uniqueness constraints to maintain data integrity and prevent incorrect data from being stored.

Database design guidelines generally recommend limiting foreign keys to non-null values ​​and ensuring uniqueness where appropriate to maintain data consistency and integrity.

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