As your applications grow and requirements change, your database schema must evolve to support new features, data structures, and business rules. Managing these changes without disrupting existing applications or losing data is one of the core challenges in database design. This process is known as schema versioning. By adopting robust schema versioning practices, you ensure that your database can adapt over time while maintaining backward compatibility, minimizing downtime, and reducing the risk of errors during upgrades.

Backward compatibility is critical: it allows older applications to continue functioning even as the schema changes. This is especially important in environments where multiple applications or services interact with the same database, or where deployments are staggered and not all codebases are updated simultaneously. Without careful versioning, introducing a breaking change—such as removing or altering a column used by existing applications—can cause failures and data integrity issues.

-- Add a new column 'stock_quantity' to the products table
ALTER TABLE products
ADD COLUMN stock_quantity INT DEFAULT 0;

-- Migrate existing data: set stock_quantity for products based on business logic
UPDATE products
SET stock_quantity = 50
WHERE category = 'Electronics';

UPDATE products
SET stock_quantity = 100
WHERE category = 'Home Appliances';

There are several strategies you can use to evolve your schema safely. The most common approach is to make additive, non-breaking changes—such as adding new columns or tables—rather than removing or renaming existing ones. Additive changes allow new features to be implemented without impacting current functionality. For example, adding a stock_quantity column to the products table enables inventory tracking, while existing applications that do not use this column continue to work as before.

When making more complex changes, such as restructuring tables or removing columns, you should plan for a transition period. This may involve supporting both old and new structures simultaneously, migrating data in stages, or providing fallback mechanisms. Always document schema versions and maintain a record of applied migrations, as seen in the schema_versions table in your database. This history is essential for troubleshooting, auditing, and rolling back changes if necessary.

-- Roll back the previous schema change by removing the 'stock_quantity' column
ALTER TABLE products
DROP COLUMN stock_quantity;