Data, Processes, and Events: Keys to Comprehensive Analysis - High-Performance Web Databases: Design, Development, and Deployment - page 51

Exhibit 4-5. Entity-relationship diagram of an order entry system.

best price of a product and its vendor. When the availability of a product is
updated in the VENDOR entity, the summary of product availability is also
updated in the PRODUCT entity.

The system must record special handling and shipping
instructions for a small percentage of orders. Often, special shipping
instructions are canceled by the customer after the order has been taken.
In such a case, an update to one entity causes the deletion of an occurrence
of another. When special shipping instruction information is removed from
the ORDER entity, the relationship with the SHIPPING INSTRUCTIONS
entity changes, and that instance of the SHIPPING INSTRUCTIONS entity
involved in the old relationship is deleted. An update to ORDER causes a
deletion of SHIPPING INSTRUCTIONS.

Update-Delete.

The ERD in Exhibit 5 shows that when an order is closed,
information about the order is recorded in the ORDER HISTORY entity.
Thus, the deletion of an instance of ORDER creates a new occurrence of
ORDER HISTORY. Obviously, the rules concerning the timing of the delete-
create event can vary by application.

Delete-Create.