1. The entity super type contains the common characteristics and the entity subtypes contain the unique characteristics of each entity subtype.
2. Entity super types and subtypes are organized in a specialization hierarchy.
3. The relationships depicted within the specialization hierarchy are sometimes described in terms of “is-a” relationships.
4. Within a specialization hierarchy, a super type can exist only within the context of a subtype.
5. One important inheritance characteristic is that all entity subtypes inherit their primary key attribute from their super type.
6. At the implementation level, the super type and its subtype(s) depicted in the specialization hierarchy maintain a 1:1 relationship.
7. Entity subtypes do not inherit the relationships in which the super type entity participates.
8. In specialization hierarchies with multiple levels of super type/subtypes, a lower-level super type inherits all of the attributes and relationships from all of its upper-level subtypes.
9. The property of subtype discriminator enables an entity super type to inherit the attributes and relationships of the subtype.
10. An entity super type can have disjoint or overlapping entity subtypes
11. Disjoint subtypes are subtypes that contain non unique subsets of the super type entity set.
12. Overlapping subtypes are subtypes that contain a unique subset of the super type entity set
13. Implementing non-overlapping subtypes requires the use of one discriminator attribute for each subtype.
14. The completeness constraint can be partial or total.
15. Specialization is the top-down process of identifying lower-level, more specific entity subtypes from a higher-level entity super type.
16. Generalization is based on grouping unique characteristics and relationships of the subtypes.
17. An entity cluster is a "virtual" entity type used to represent multiple entities and relationships in the ERD.
18. To model time-variant data, you must create a new entity in a M:N relationship with the original entity.
19. A design trap occurs when a relationship is improperly or incompletely identified and is therefore represented in a way that is not consistent with the real world.
20. Some designs use redundant relationships as a way to simplify the design.
