SQL SERVER – Curious Case of Disappearing Rows – ON UPDATE CASCADE and ON DELETE CASCADE – Part 1 of 2

Social media has created an Always Connected World for us. Recently, I noticed a notification storm on my social media handles, with messages pouring in from platforms like Twitter, Facebook, LinkedIn, and even emails and WhatsApp messages. One of the issues raised caught my attention, and it revolved around a puzzling situation related to ON UPDATE CASCADE and ON DELETE CASCADE in SQL Server.

The problem reported was unique: rows in a product details table were continuously being deleted without an apparent DELETE command being executed. Despite removing all references to the table from the application code and extensive troubleshooting, rows were still disappearing in a random pattern. This led to concerns about potential intrusion or a virus.

The issue seemed urgent, so I got online to investigate further. After checking audit logs, policy management, and profiling the data, it became clear that the problem was not as advanced as initially feared. There was no intrusion, SQL Injection, or virus. Instead, the root cause was a simple but impactful design decision: foreign keys created with ON UPDATE CASCADE and ON DELETE CASCADE.

What Are ON UPDATE CASCADE and ON DELETE CASCADE?

CASCADE allows updates or deletions in a table to propagate automatically to other related tables that have foreign key constraints. Here’s how it works:

• ON DELETE CASCADE: If a row with a primary key is deleted, all rows in related tables with foreign keys referencing that key are also deleted.
• ON UPDATE CASCADE: If a key value in a row is updated, all foreign key values in related tables are updated to the new value.

For example, when data is deleted from Table A and Table B references Table A with a foreign key, the data in Table B will also be deleted if ON DELETE CASCADE is specified. (Reference: BOL)

Real-Life Scenario: What Went Wrong

In this case, there were two tables: Products and ProductDetails. A foreign key was defined between them on the product ID column. During an update to the catalog, products that were no longer available were deleted from the Products table. Due to ON DELETE CASCADE, corresponding rows in the ProductDetails table were also deleted automatically. This behavior was by design and completely correct from SQL Server’s perspective. However, the issue arose because of a lack of understanding and inappropriate implementation of business logic.

The proper solution would have been to implement a Product Master Table, a Current Product Catalogue, and a Product Order Details History Table. This separation would ensure proper management of business logic without accidental cascading deletions. Alternatively, a soft delete approach could have been used, marking records as inactive instead of actually deleting them. This would have prevented unintended cascading deletions.

Lessons Learned

This situation highlights the importance of understanding database design and foreign key constraints. SQL Server was behaving as expected, but the inappropriate implementation of business logic led to unintended consequences. Sometimes, we are our own enemy, and this was a prime example of it.

In tomorrow’s blog post, I will go over the code involved and discuss possible workarounds for such scenarios. Feel free to share your opinions, experiences, and comments.

You can connect with me on LinkedIn.

Reference: Pinal Dave (https://blog.sqlauthority.com)