next up previous
Next: Design of an Up: The Entity-Relationship Model Previous: Generalization


The E-R model cannot express relationships among relationships.

When would we need such a thing?

Consider a DB with information about employees who work on a particular project and use a number of machines doing that work. We get the E-R diagram shown in Figure 2.20.

Figure 2.20: E-R diagram with redundant relationships

Relationship sets work and uses could be combined into a single set. However, they shouldn't be, as this would obscure the logical structure of this scheme.

The solution is to use aggregation.

Transforming an E-R diagram with aggregation into tabular form is easy. We create a table for each entity and relationship set as before.

The table for relationship set uses contains a column for each attribute in the primary key of machinery and work.

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