Subclasses, Superclasses, and Inheritance




SUBCLASS AND SUPERCLASS

An entity type is used to represent both a type of entity and the entity set or collection of entities of that type that exist in the database.

For example, the entity type EMPLOYEE describes the type (that is, the attributes and relationships) of each employee entity, and also refers to the current set of EMPLOYEE entities in the COMPANY database. In many cases an entity type has numerous subgroupings or subtypes of its entities that are meaningful and need to be represented explicitly because of their significance to the database application.

For example, the entities that are members of the EMPLOYEE entity type may be distinguished further into SECRETARY, ENGINEER, MANAGER, TECHNICIAN, SALARIED_EMPLOYEE, HOURLY_EMPLOYEE, and so on. The set of entities in each of the latter groupings is a subset of the entities that belong to the EMPLOYEE entity set, meaning that every entity that is a member of one of these subgroupings is also an employee.We call each of these subgroupings a subclass or subtype of the EMPLOYEE entity type, and the EMPLOYEE entity type is called the superclass or supertype for each of these subclasses. Figure 8.1 shows how to represent these concepts diagramatically in EER diagrams. (The circle notation in Figure 8.1 will be explained in Section 8.2.) We call the relationship between a superclass and any one of its subclasses a superclass/subclass or supertype/subtype or simply class/subclass relationship.3 In our previous example, EMPLOYEE/SECRETARY and EMPLOYEE/TECHNICIAN are two class/subclass relationships. Notice that a member entity of the subclass represents the same real-world entity as some member of the superclass; for example, a SECRETARY entity ‘Joan Logano’ is also the EMPLOYEE ‘Joan Logano.’ Hence, the subclass member is the same as the entity in the superclass, but in a distinct specific role.When we implement a superclass/subclass relationship in the database system,however, we may represent a member of the subclass as a distinct database object say, a distinct record that is related via the key attribute to its superclass entity. In Section 9.2, we discuss various options for representing superclass/subclass relationships in relational databases.

 

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Subclasses, Superclasses, and Inheritance

 

 

 

 

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INHERITANCE

An entity cannot exist in the database merely by being a member of a subclass; it must also be a member of the superclass. Such an entity can be included optionally as a member of any number of subclasses. For example, a salaried employee who is also an engineer belongs to the two subclasses ENGINEER and SALARIED_EMPLOYEE of the EMPLOYEE entity type. However, it is not necessary that every entity in a superclass is a member of some subclass. An important concept associated with subclasses (subtypes) is that of type inheritance.Recall that the type of an entity is defined by the attributes it possesses and the relationship types in which it participates. Because an entity in the subclass represents the same real-world entity from the superclass, it should possess values for its specific attributes as well as values of its attributes as a member of the superclass.
We say that an entity that is a member of a subclass inherits all the attributes of the entity as a member of the superclass. The entity also inherits all the relationships in which the superclass participates. Notice that a subclass, with its own specific (or local) attributes and relationships together with all the attributes and relationships it inherits from the superclass, can be considered an entity type in its own right.

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Frequently Asked Questions

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Ans: A database system is a collection of interrelated data and a set of programs that allow users to access and modify these data. A major purpose of a database system is to provide users with an abstract view of the data. That is, the system hides certain details of how the data are stored and maintained. view more..
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Ans: Self-describing nature of a database system Insulation between programs and data, and data abstraction Support of multiple views of the data Sharing of data and multiuser transaction processing view more..
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Ans: The specialization relationship may also be referred to as a superclass-subclass relationship. Higher- and lower-level entity sets also may be designated by the terms superclass and subclass, respectively. The person entity set is the superclass of the employee and student subclasses. view more..
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Ans: A database-management system (DBMS) is a collection of interrelated data and a set of programs to access those data. view more..
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Ans: Databases are widely used in enterprises, banking and finance, universities, airlines, telecommunication, etc. view more..
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Ans: Specialization is the process of defining a set of subclasses of an entity type; this entity type is called the superclass of the specialization. We use the term generalization to refer to the process of defining a generalized entity type from the given entity types. view more..
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Ans: we discuss differences between specialization/generalization lattices (multiple inheritance) and hierarchies (single inheritance), and elaborate on the differences between the specialization and generalization processes during conceptual database schema design. We discuss constraints that apply to a single specialization or a single generalization. view more..
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Ans: it is sometimes necessary to represent a single superclass/subclass relationship with more than one superclass, where the superclasses represent different entity types. In this case, the subclass will represent a collection of objects that is a subset of the UNION of distinct entity types. view more..
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Ans: For our sample database application, consider a UNIVERSITY database that keeps track of students and their majors, transcripts, and registration as well as of the university’s course offerings. The database also keeps track of the sponsored research projects of faculty and graduate students. view more..
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Ans: The basic notation for specialization/generalization is to connect the subclasses by vertical lines to a horizontal line, which has a triangle connecting the horizontal line through another vertical line to the superclass. A blank triangle indicates a specialization/generalization with the disjoint constraint, and a filled triangle indicates an overlapping constraint. view more..
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Ans: The similarities and differences between conceptual modeling and knowledge representation, and introduces some of the alternative terminology and a few additional concepts.The goal of KR techniques is to develop concepts for accurately modeling some domain of knowledge by creating an ontology that describes the concepts of the domain and how these concepts are interrelated. view more..
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Ans: Recovery from transaction failures usually means that the database is restored to the most recent consistent state just before the time of failure. To do this, the system must keep information about the changes that were applied to data items by the various transactions. view more..
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Ans: Several types of locks are used in concurrency control. To introduce locking concepts gradually, first we discuss binary locks, which are simple, but are also too restrictive for database concurrency control purposes, and so are not used in practice. Then shared/exclusive locks - also known as read/write locks - which provide more general locking capabilities and are used in practical database locking systems. view more..
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Ans: In this section we discuss the concepts of concurrent execution of transactions and recovery from transaction failures. view more..
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Ans: Conceptual modeling is a very important phase in designing a successful database application. Generally, the term database application refers to a particular database and the associated programs that implement the database queries and updates. view more..
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Ans: Data is converted into information, and information is then evaluated and organised so that it can be used purposefully as knowledge. view more..
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Ans: A database system is a collection of interrelated data and a set of programs that allow users to access and modify these data. A major purpose of a database system is to provide users with an abstract view of the data. That is, the system hides certain details of how the data are stored and maintained. view more..




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