Add Foreign Key relationship between two Databases

sql

data-integrity

database-design

triggers

byAlex Kataev·Oct 10, 2024

Simulating a foreign key between two databases can be achieved through creating a trigger to ensure referential integrity, or utilizing a cross-database reference in SQL Server.

For instance, a cross-database foreign key in SQL Server could look something like this:

-- "Let's make sure the new record will relate to existing one, shall we?" 
ALTER TABLE Database1.dbo.Table1
ADD CONSTRAINT FK_Table1_Table2
FOREIGN KEY (Column1) REFERENCES Database2.dbo.Table2(Column1);

If you want a method that works across different DBMS, consider using a trigger:

-- "If this record doesn't have a buddy in the other database, we'll get an earful!"
CREATE TRIGGER CheckForeignKey
ON Database1.dbo.Table1
FOR INSERT, UPDATE 
AS
BEGIN
    IF NOT EXISTS (
        SELECT 1 FROM Database2.dbo.Table2 
        WHERE Column1 = INSERTED.Column1
    )
    RAISERROR ('Foreign key violation', 16, 1); --"Oops, someone's going to get a violation sticker!"
    ROLLBACK;
END;

Keep in mind, there are risks associated with these methods such as data recovery challenges and potential performance impacts.

Trading Off: Benefits vs Limitations of Cross-Database Foreign Keys

While employing cross-database foreign keys ensures data integrity across separate databases, it introduces a set of constraints. This includes the difficulties in maintaining such relationships due to backups & restorations, as well as varying permissions between databases. Furthermore, potential performance slowdowns could be an unwanted side effect as well as an increase in system's overall complexity.

Surviving Refusals: Dealing with DBMS Restrictions

Most DBMS systems pose restrictions which can hinder implementing cross-database foreign keys directly. In such cases, triggers can create an illusion of foreign key's actions. However, it's crucial to ensure the trigger simulates the foreign key constraint precisely, including potential cascading deletes or updates. Triggers do come with a baggage of being harder to debug and having possibility of transaction delays.

Crisis Management: When to Use Triggers for Referential Integrity?

When cross-database foreign key constraints are not viable, triggers come up as a flexible solution. Common scenarios might include databases hosted in different environments, cross-platform data integrity requirements, or when the DBMS doesn't natively support cross-database integrity. While triggers can serve as valuable tools, they require careful design and testing due to their complexity.

Plan B: Alternatives to Cross-Database Foreign Keys

If the complexity of triggers or cross-database references is not ideal, consider refactoring the database design:

Consolidate databases: Move related tables into the same database, in order to avoid cross-database references.
Rely on applications: Enforce data integrity in the application layer as opposed to the database layer. However, this shifts the responsibility to the developers and may be less reliable.
Replicate: Use replication or change data capture to synchronize data across different databases. Although, this introduces computation overhead.

Orphan-Watch: Managing Orphaned Records

In the absence of database-level integrity assurance, handling orphaned records becomes paramount. Automate regular consistency checks, and establish cleanup procedures for orphaned records. Note, triggers do not inherently manage cleanup. Thus, custom scripts or scheduled tasks might be necessary to keep your databases consistent and free of orphans.