We have seen weak entity sets, generalization and aggregation. Designers must decide when these features are appropriate.

• Strong entity sets and their dependent weak entity sets may be regarded as a single \object" in the database,as weak entities are existence-dependent on a strong entity.
• It is possible to treat an aggregated entity set as a single unit without concern for its inner structure details.
• Generalization contributes to modularity by allowing common attributes of similar entity sets to be repre-sented in one place in an E-R diagram. Excessive use of the features can introduce unnecessary complexity into the design.