Chp 2 Last Part.pdf

Entity-Relationship (E/R) Model Generalisation, Specialisation & aggregation Prof. Jugal J Manek VLSI Dept, Shah & Anchor Kutchhi Engineering College  Extended ER Features ▪ Requirement. ▪ A) Specialization ▪ B) Generalization ▪...

Entity-Relationship (E/R) Model Generalisation, Specialisation & aggregation Prof. Jugal J Manek VLSI Dept, Shah & Anchor Kutchhi Engineering College  Extended ER Features ▪ Requirement. ▪ A) Specialization ▪ B) Generalization ▪ C) Aggregation 2  Specialization An entity set may include subgroupings of entities that are distinct in some way from other entities in the set. For instance, a subset of entities within an entity set may have attributes that are not shared by all the entities in the entity set. Example: Graduate students have an office assigned to them. Undergraduate students are assigned to a residential college. Each of these student types is described by a set of attributes that includes all the attributes of the entity set student plus additional attributes. 3  Specialization The university could create two specializations of students, namely graduate and undergraduate. The entity set graduate would have all the attributes of student and an additional attribute office number. The entity set undergraduate would have all the attributes of student, and an additional attribute residential college. 4 Specialization 5  Specialization In terms of an E-R diagram, specialization is depicted by a hollow arrow-head pointing from the specialized entity to the other entity. We refer to this relationship as the ISA relationship, which stands for “is a” and represents, for example, that an instructor “is a” employee. Also know as Top-Down approach. 6  Generalization Also known as Bottom Up Approach. In which multiple entity sets are synthesized into a higher-level entity set on the basis of common features. Say we have, 7  Generalization There are similarities between the instructor entity set and the secretary entity set in the sense that they have several attributes that are conceptually the same across the two entity sets: namely, the identifier, name, and salary attributes. This commonality can be expressed by generalization To create a generalization, the attributes must be given a common name and represented with the higher-level entity person. 8  Aggregation One limitation of the E-R model is that it cannot express relationships among relationships. Aggregation is an abstraction through which relationships are treated as higher-level entities. For Example, an Employee working on a project may require some machinery. So, REQUIRE relationship is needed between the relationship WORKS_FOR and entity MACHINERY. 9  Aggregation Using aggregation, WORKS_FOR relationship with its entities EMPLOYEE and PROJECT is aggregated into a single entity and relationship REQUIRE is created between the aggregated entity and MACHINERY. 10 11