E-commerce order system ER diagram modeling a polymorphic Order entity with StandardOrder and SubscriptionOrder subtypes, each with distinct fulfillment logic. Orders connect to Customers, OrderItems (which reference Products), Payments, and an OrderStatus enumeration, with abstract methods for total calculation and validation. This inheritance-based design separates one-time and recurring billing workflows while maintaining shared order attributes like customerId, createdAt, and status. The diagram demonstrates how to structure complex transactional systems with clear entity relationships, foreign keys, and business logic contracts. Fork and customize this template on Diagrams.so to adapt it for your platform's payment processing, inventory, and subscription requirements.