Data, Processes, and Events: Keys to Comprehensive Analysis - High-Performance Web Databases: Design, Development, and Deployment - page 50

Exhibit 4-4. The nine combinations of tight coupling.

ITEM. Not only is this a full relationship but, more specific, the creation of
an occurrence of one entity causes the creation of an occurrence of
another entity. The creation of an ORDER does not just establish a relation-
ship with an existing occurrence of LINE ITEM, it creates a new occurrence
of LINE ITEM. In this tightly coupled relationship, the create state in one
entity causes a create state in another entity.

Exhibit 5 shows that CUSTOMER has many INVOICES.
The CUSTOMER entity has a data element called Date of Last Invoice. Thus,
a new occurrence (i.e., creation) of INVOICE causes Date of Last Invoice to
be updated.

Create-Update.

In the order-entry system, a customer who fails to pay
after a certain period is placed in a bad customer file, which is used by the
collections department. Such a customer is no longer allowed to conduct
business with the supplier. The ERD in Exhibit 5 illustrates that the creation
of an occurrence of a BAD CUSTOMER entity causes an instance of the CUS-
TOMER entity to be deleted.

Create-Delete.

The order-entry system must handle price changes,
which can be volatile. The PRODUCT entity has a data element that stores
the latest price, and for each price change a new instance of the PRICE
CHANGE JOURNAL entity records the date, time, person changing, and old
and new prices. In other words, an update in the PRODUCT entity causes
the creation of a new occurrence of PRODUCT CHANGE JOURNAL.

Update-Create.

The system must store the price and availability of all
products from every vendor. The PRODUCT entity must always reflect the

Update-Update.