Diagram data objects
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. IBM®
Informix® 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. Figure 1 shows
a sample E-R diagram.
In an E-R diagram, a box represents an entity.
A line represents the relationships that connect the entities. In
addition, Figure 2 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.