Extended ER is a high-level data model that incorporates the extensions to the original ER model. Enhanced entity-relationship diagrams are advanced database diagrams very similar to regular ER diagrams which represent the requirements and complexities of complex databases.
Enhanced ER Model
concepts are:
- Generalization
- Specialization
- Aggregation
Generalization
Generalization is a
bottom-up approach in which two or more entities can be generalized to a
higher-level entity if they have some attributes in common.
It is the process of
extracting common properties from a set of entities and creating a generalized
entity from it. This process combines a number of entity sets that share the
same features into higher-level entity sets.
Generalization is used
to emphasize the similarities among the lower level entity sets and to hide the
differences.
Specialization
In specialization, an
entity is divided into sub-entities based on its characteristics. It is a
top-down approach where the higher-level entity is specialized into two or more
lower-level entities.
The process of
designing sub groupings within an entity set is called specialization. Lower
level entity sets inherits all the attributes and relationship of the higher
level entity set to which it is linked.
Specialization can be
depicted in E-R diagram by a hallow arrow-head pointing from specialized entity
to the other entity.
Aggregation
Aggregation allows us
to indicate that a relationship set participates in another relationship set. It
is an abstraction in which relationship sets are treated as higher level entity
sets and can participate in relationships.
Aggregation allows
relationships between relationships. Aggregation treat relationship as an
abstract entity.
No comments:
Post a Comment