Why should you care?

Bad early design in a SaaS billing module forces painful migrations when GST rules change nationwide.

See it live — copy this example

Run in MySQL Workbench or the mysql CLI.

CREATE TABLE order_items (
  line_id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  order_id INT UNSIGNED NOT NULL,
  product_id INT UNSIGNED NOT NULL,
  qty INT NOT NULL,
  unit_price_inr DECIMAL(10,2) NOT NULL,
  FOREIGN KEY (order_id) REFERENCES orders(order_id),
  FOREIGN KEY (product_id) REFERENCES products(product_id)
);

What happened?

• Prices snapshotted in unit_price_inr so historical invoices stay correct if catalog price changes later.
• FKs keep links honest.

Practice next

1. Sketch entities: customer, order, line, product.
2. Assign PK and FK for each arrow.
3. build in Workbench EER diagram.
4. Add status ENUM on orders for workflow.
5. Document denormalized cache table vs live JOIN tradeoff.

Remember

Design for queries you will run. Snapshot prices on immutable facts. Normalize first; denormalize with reason.

DataFlow MVP schema

Team ships four tables instead of forty — enough for checkout and admin.

Outcome: Launch on time; extend schema incrementally.