DBMS E-R MODEL
Introduction
Peter Chen first proposed modeling databases using a graphical technique that humans can relate to easily. Humans can easily perceive entities and their characteristics in the real world and represent any relationship with one another. The objective of modeling graphically is even more profound than simply representing these entities and relationship. The database designer can use tools to model these entities and their relationships and then generate database vendor-specific schema automatically. Entity–Relationship (ER) model gives the conceptual model of the world to be represented in the database. ER Model is based on a perception of a real world that consists of collection of basic objects called entities and relationships among these objects. The main motivation for defining the ER model is to provide a high level model for conceptual database design, which acts as an intermediate stage prior to mapping the enterprise being modeled onto a conceptual level. The ER model achieves a high degree of data independence which means that the database designer do not have to worry about the physical structure of the database. A database schema in ER model can be pictorially represented by Entity–Relationship diagram.
The Building Blocks of an Entity–Relationship Diagram
ER diagram is a graphical modeling tool to standardize ER modeling. The modeling can be carried out with the help of pictorial representation of entities, attributes, and relationships. The basic building blocks of Entity- Relationship diagram are Entity, Attribute and Relationship.
Entity:
An entity is an object that exists and is distinguishable from other objects. In other words, the entity can be uniquely identified.
The examples of entities are:
– A particular person, for example Dr. A.P.J. Abdul Kalam is an entity.
– A particular department, for example Electronics and Communication Engineering Department.
– A particular place, for example Coimbatore city can be an entity.
Entity Type
An entity type or entity set is a collection of similar entities. Some examples of entity types are:
– All students in PSG, say STUDENT.
– All courses in PSG, say COURSE.
– All departments in PSG, say DEPARTMENT.
An entity may belong to more than one entity type. For example, a staff working in a particular department can pursue higher education as part-time. Hence the same person is a LECTURER at one instance and STUDENT at another instance.
Relationship
A relationship is an association of entities where the association includes one entity from each participating entity type whereas relationship type is a meaningful association between entity types.
The examples of relationship types are:
– Teaches is the relationship type between LECTURER and STUDENT.
– Buying is the relationship between VENDOR and CUSTOMER.
– Treatment is the relationship between DOCTOR and PATIENT.
Attributes
Attributes are properties of entity types. In other words, entities are described in a database by a set of attributes.
Enhanced Entity–Relationship Model (EER Model):
The basic concepts of ER modeling are not powerful enough for some complex applications. Hence some additional semantic modeling concepts are required, which are being provided by Enhanced ER model. The Enhanced ER model is the extension of the original ER model with new modeling constructs. The new modeling constructs introduced in the EER model are supertype (superclass)/
subtype (subclass) relationships. The supertype allows us to model general entity type whereas the subtype allows us to model specialized entity types.
Enhanced ER model = ER model + hierarchical relationships.
EER modeling is especially useful when the domain being modeled is object-oriented in nature and the use of inheritance reduces the complexity of the design. The extended ER model extends the ER model to allow various types of abstraction to be included and to express constraints more
clearly.
Supertype or Superclass
Supertype or superclass is a generic entity type that has a relationship with one or more subtypes. For example PLAYER is a generic entity type which has
a relationship with one or more subtypes like CRICKET PLAYER, FOOTBALL
PLAYER, HOCKEY PLAYER, TENNIS PLAYER, etc.
Subtype or Subclass
A subtype or subclass is a subgrouping of the entities in an entity type that is meaningful to the organization. A subclass entity type is a specialized type of superclass entity type. A subclass entity type represents a subset or subgrouping of superclass entity type’s instances. Subtypes inherit the attributes and relationships associated with their supertype.
Consider the entity type ENGINE, which has two subtypes PETROL ENGINE and DIESEL ENGINE.
Consider the entity type STUDENT, which has two subtypes UNDERGRADUATE and POSTGRADUATE.
Generalization and Specialization
Generalization and specialization are two words for the same concept, viewed from two opposite directions. Generalization is the bottom-up process of defining a generalized entity type from a set of more specialized entity types. Specialization is the top-down process of defining one or more subtypes of a supertype. Generalization is the process of minimizing the differences between entities by identifying common features. It can also be defined as the process of defining a generalized entity type from a set of entity types. Specialization is a process of identifying subsets of an entity set (the superset) that share some distinguishing characteristics. In specialization the superclass is defined first and the subclasses are defined next. Specialization is the process of viewing an object as a more refined, specialized object. Specialization emphasizes the differences between objects. For example consider the entity type STUDENT, which can be further classified into FULLTIME STUDENT and PARTTIME STUDENT. The classification of STUDENT into FULLTIME STUDENT and PARTTIME STUDENT is called Specialization.
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