Other Styles of E-R Diagram



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Next: Reducing E-R Diagrams Up: The Entity-Relationship Model Previous: The Entity Relationship

Other Styles of E-R Diagram

The text uses one particular style of diagram. Many variations exist.

Some of the variations you will see are:

Roles in E-R Diagrams

The function that an entity plays in a relationship is called its role. Roles are normally explicit and not specified.

They are useful when the meaning of a relationship set needs clarification.

For example, the entity sets of a relationship may not be distinct. The relationship works-for might be ordered pairs of employees (first is manager, second is worker).

In the E-R diagram, this can be shown by labelling the lines connecting entities (rectangles) to relationships (diamonds). (See figure 2.11).

  
Figure 2.11: E-R diagram with role indicators

Weak Entity Sets in E-R Diagrams

A weak entity set is indicated by a doubly-outlined box. For example, the previously-mentioned weak entity set transaction is dependent on the strong entity set account via the relationship set log.

Figure 2.12) shows this example.

  
Figure 2.12: E-R diagram with a weak entity set

Nonbinary Relationships

Non-binary relationships can easily be represented. Figure 2.13) shows an example.

  
Figure 2.13: E-R diagram with a ternary relationship

This E-R diagram says that a customer may have several accounts, each located in a specific bank branch, and that an account may belong to several different customers.



next up previous
Next: Reducing E-R Diagrams Up: The Entity-Relationship Model Previous: The Entity Relationship



Page created and maintained by Osmar R. Zaï ane
Last Update: Sun Sep 10 17:02:35 PDT 1995