Data, Processes, and Events: Keys to Comprehensive Analysis - High-Performance Web Databases: Design, Development, and Deployment - page 49

Exhibit 4-3. Existence criteria in an entity-relationship diagram.

EXISTENCE CRITERIA AND COUPLING

When examining a relationship between two entities, analysts must con-
sider the relationship's cardinality and whether the relationship is
required or optional for each entity. Exhibit 3 depicts existence criteria.
The O symbol indicates that the relationship is optional, and the I symbol
indicates that the relationship is required. Because all relationships are
bidirectional, the analyst must show the existence criteria for each direc-
tion (i.e., entity). Exhibit 3 models the fact that a course must have an
instructor. The two bars in the diagram indicate that a maximum of one
instructor can teach a course. The O next to the COURSE entity indicates
that an instructor does not have to teach any course.

In addition to existence criteria, the coupling of a relationship must also
be examined. Coupling measures how much an action (i.e., change in
stage) in one entity affects actions in a related entity. When assessing the
coupling and existence criteria of a relationship, analysts must also deter-
mine whether the relationship is full or sparse. A relationship is considered
full if the new occurrence of an entity immediately establishes a relation-
ship with at least one occurrence of another entity. A full relationship is a
form of moderate coupling, but this view of a relationship does not com-
pletely depict the nature of relationships.

In tightly coupled relationships, actions in one entity cause actions in
another. There are nine possible combinations of a tightly coupled relation-
ship. These nine possibilities are the product of combining create, update,
and delete states in two entities (see Exhibit 4). The nine types of tightly
coupled relationships are examined in the next section.

Types of Tightly Coupled Relationships

The following sections illustrate cases where action in one entity causes
an action in another.

Exhibit 5 models an order-entity and billing system. A
relationship exists between ORDER and LINE ITEMS. The ERD in Exhibit 5
shows that the relationship is required on both sides; that is, an order must
have at least one line item and a line item must be associated with an order.

Create-Create.

The relationship between these two entities has another dimension. The
creation of an ORDER immediately establishes a relationship with a LINE