Explain the concept of generalization and aggregation in E_R diagrams. Give one example for each one of them?

Generalization: Consider extending the entity set account through classifying accounts as being either savings-account or chequing-account.
Each of these is described through the attributes of account plus additional attributes. (Savings has interest-rate and cheque has overdraft-amount.)
We can express the similarities among the entity sets through generalization. This is the procedure of forming containment relationships among a higher-level entity set and one or more lower-level entity sets.

Aggregation: The E-R model cannot express relationships between relationships.
When would we require such a thing?

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

Figure: E-R diagram with redundant relationships

Relationship sets work and uses could be combined into a single set. Therefore, they shouldn't be, as this would obscure the logical structure of this scheme.
The solution is to use aggregation.
• An abstraction by those relationships is treated as higher-level entities.
• For our instance, we treat the relationship set work and the entity sets employee and project as a higher-level entity set called work.
• Another figure shows the E-R diagram with aggregation.