Chapter 3, Problem 3.1RQ

Define each of the following terms:

1. supertype
2. subtype
3. specialization
4. entity cluster
5. completeness constraint
6. enhanced entity-relationship (EER) model
7. supertype/subtype hierarchy
8. total specialization rule
9. generalization
10. disjoint rule
11. overlap rule
12. partial specialization rule
13. universal data model

Program Plan Intro

(a)

Definition of supertype

Explanation of Solution

Supertype is an entity that has relationship with one or more subtypes and contains some common subtype attributes. For example, when we are designing a data model for the employee, then we can have employee as a supertype, and its attributes like salary employee and contracted employee are taken as subtype.

Program Plan Intro

(b)

Subtype

Explanation of Solution

Subtypes are the subgroups of the supertype entities. Each subtype consists of some unique attributes and is different from each other. For example, when we are creating a data model for the employee details, here we have one supertype entity employee with many subtype entities like part time employee, full time employee, and salaried employee, etc.

Program Plan Intro

(c)

Specialization

Explanation of Solution

It is an opposite approach of the generalization. It is used to break down the higher level of the entity into the subgroups of lower level entity. Specialization is used to identify the subset entity that is sharing some common and distinguished characters.

Example:

Program Plan Intro

(d)

Entity cluster

Explanation of Solution

Entity cluster is a useful way to represent the data model for large and complex organization. Entity cluster is a collection of various entities that are combined to form one common entity. An entity can be considered as virtual in the entity cluster. These entities are developed with the purpose of the reliability and simplification of data. EER diagram of an entity cluster is given below:

Program Plan Intro

(e)

Completeness constraints

Explanation of Solution

Completeness constraints are used to check that the supertype entity must have an occurrence of the subtype entity. It is basically used to check the common attributes among the supertype and the subtype. There are two types of completeness constraints:

1. Partial specialization: In Partial specialization, it is not necessary that all the subtype and the supertype entities are related to each other totally. There might be some cases where a partial relationship between the entities is possible. It can be represented by using a single line. Consider the below given diagram.



Here, a person has a license. Suppose we have 5 people among which only 2 have license and rest do not. Then, it is a case of partial specialization. Here, it not necessary that all the people have license, there might be some persons who do not possess a license.

2. Total specialization: In total specialization, it is necessary that all the subtype and the supertype entities are related to each other totally. It can be represented by using a single line. Consider the below given diagram.



Here, the license should always belong to a particular person, i.e., there is no license without a person. This is a case of total specialization.

Program Plan Intro

(f)

Enhanced entity-relationship (EER) model.

Explanation of Solution

Enhanced entity-relationship (EER) model is the enhanced model form of entity relational data model. It is a higher-level conceptual model of the computer science that is used to develop the advance databases, complex software designs, and geographic information systems (GIS), etc. Enhanced entity-relationship (EER) model reflects the data properties and constraints more precisely. It also consists of all the concepts of the entity relation diagram, specialization, and generalization.

Program Plan Intro

(g)

Supertype/subtype hierarchy.

Explanation of Solution

Supertype/subtype hierarchy is the structure in which the supertype and the subtype entities are leveled according to their order. This organization of data make the data more modular and easier to understand and use.

Program Plan Intro

(h)

Total specialization rule

Explanation of Solution

Total specialization rule is used to ensure that every entity of the subtype belongs to the supertype entity, i.e., there must be no entity of superclass which is not belongs to the subtype.

Program Plan Intro

(i)

Generalization.

Explanation of Solution

Generalization is a process of extracting common features from multiple entities source in order to create a new entity. it helps in reducing the size of schema. Generalization provide one entity from multiple entity. Bottom − up designing strategies are used in the Generalization.

In Generalization entity all the all higher-level entities have the lower level entity. There is no higher entity without the lower level entity.

Program Plan Intro

(j)

Disjoint rules

Explanation of Solution

The disjoint rule specifies that the entity object of supertype can only be the member of any of the subtype. For example, let’s consider animal as a supertype. Then it can have subtype entities like dog, cat, etc., In disjoint rule, the supertype class animal can be associated with the dog or cat at one time. i.e., the animal can be either dog or cat.

Program Plan Intro

(k)

Overlap rule

Explanation of Solution

In the overlap rule, the supertype can be associated with more than one subtype entities. For example, consider a supertype entity student that can be associated with many subjects like math, English, Hindi and many more.

Program Plan Intro

(l)

Partial specialization rules

Explanation of Solution

Partial specialization rules allow that the entity of supertype must not always need to belong to the subtype entity. Consider a supertype entity vehicle having subtypes entities car and truck. Here motorcycle is also subtype of vehicles, but it is not specified as subtype in the data model. Thus, if a vehicle is a car, then it must appear in the object of car and if the vehicle is a truck, it must appear in the object of truck. But if the vehicle is a motorcycle, then it cannot appear in any of the subtypes.

Program Plan Intro

(m)

Universal data model

Explanation of Solution

Universal data model can be used as a starting point for the data modeling project. It is also known as a data model pattern. Advantages of using the Universal data model:

• Project takes less time and efforts to develop as all the essential components and structures are already designed and need only to be customized according to the requirement.
• Data models of the existing database are easier to read by the data modeler and other data management experts, as it is based on the common components seen in the other situations.