Now you know and understand
the entities and relationships in your database, which is the most
important part of the relational-database design process. After you
determine the entities and relationships, a method that displays your
thought process during database design might be helpful.
Most
data-modeling methods provide some way to graphically display the
entities and relationships. HCL®
OneDB® documentation
uses the E-R diagram approach that C. R. Bachman originally developed.
E-R diagrams serve the following purposes. They:
Model the
informational requirements of an organization
Identify entities
and their relationships
Provide a starting point for data
definition (data-flow diagrams)
Provide an excellent source
of documentation for application developers and both database and
system administrators
Create a logical design of the database
that can be translated into a physical schema
Several
different styles of E-R diagrams exist. If you already have a style
that you prefer, use it. Symbols
of an Entity-Relationship diagram shows
a sample E-R diagram. Figure 1: Symbols
of an Entity-Relationship diagram
In an E-R diagram, a box represents an entity.
A line represents the relationships that connect the entities. In
addition, The parts of a relationship
in an Entity-Relationship diagram shows how
you use graphical items to display the following features of relationships:
A circle across a relationship link indicates optionality in
the relationship (zero instances can occur).
A small bar across
a relationship link indicates that exactly one instance of the entity
is associated with another entity (consider the bar to be a 1).
The
crow's-feet represent many in the relationship. Figure 2: The parts of a relationship
in an Entity-Relationship diagram
