Aggregation

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.

• An abstraction through which relationships are treated as higher-level entities.
• For our example, we treat the relationship set work and the entity sets employee and project as a higher-level entity set called work.

• Figure 2.21 shows the E-R diagram with aggregation.

    
  Figure 2.21: E-R diagram with aggregation
  

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