Diagramming 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 needs 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 as well as 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 12 shows a sample E-R diagram.

Figure 12. 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, Figure 13 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 13. The Parts of a Relationship in an Entity-Relationship Diagram