The Completeness Constraint Can Be Partial or Total.
Constraints and Characteristics of Specialization and Generalization Hierarchies
In general, nosotros may have several specializations divers on the same entity type (or superclass), as shown in Figure 8.1. In such a case, entities may belong to subclasses in each of the specializations. However, a specialization may also consist of a single subclass only, such as the {MANAGER} specialization in Effigy 8.1; in such a example, nosotros exercise not employ the circumvolve annotation. In some specializations we can decide exactly the entities that will get members of each subclass by placing a status on the value of some attribute of the superclass. Such subclasses are called predicate-defined (or condition-defined) subclasses. For example, if the EMPLOYEE entity blazon has an attribute Job_type, as shown in Figure 8.4, nosotros can specify the condition of membership in the Secretarial assistant subclass past the status (Job_type = 'Secretary'), which nosotros call the defining predicate of the bracket. This condition is a constraint specifying that exactly those entities of the EMPLOYEE entity type whose aspect value for Job_type is 'Secretary' vest to the subclass.We display a predicate-divers subclass by writing the predicate status next to the line that connects the bracket to the specialization circumvolve. If all subclasses in a specialization take their membership condition on the same attribute of the superclass, the specialization itself is called an attribute-defined specialization, and the attribute is called the defining aspect of the specialization. In this case, all the entities with the same value for the attribute belong to the aforementioned subclass. Nosotros display an aspect-defined specialization by placing the defining aspect name next to the arc from the circle to the superclass, as shown in Figure 8.4. When we do not have a condition for determining membership in a subclass, the subclass is called user-defined. Membership in such a subclass is adamant by the database users when they utilize the performance to add an entity to the subclass; hence, membership is specified individually for each entity by the user, not by whatever condition that may exist evaluated automatically.
Two other constraints may apply to a specialization. The outset is the disjointness (or disjointedness) constraint, which specifies that the subclasses of the specialization must be disjoint. This means that an entity can exist a member of at well-nigh one of the subclasses of the specialization. A specialization that is attribute-defined implies the disjointness constraint (if the attribute used to define the membership predicate is single-valued). Effigy 8.4 illustrates this case, where the d in the circumvolve stands for disjoint. The d annotation also applies to user defined subclasses of a specialization that must be disjoint, every bit illustrated by the specialization {HOURLY_EMPLOYEE, SALARIED_EMPLOYEE} in Figure 8.1. If the subclasses are not constrained to be disjoint, their sets of entities may exist overlapping; that is, the same (existent-world) entity may exist a member of more one subclass of the specialization. This case, which is the default, is displayed by placing an o in the circle, equally shown in Figure 8.5.
The second constraint on specialization is chosen the completeness (or totalness) constraint, which may be total or partial. A total specialization constraint specifies that every entity in the superclass must be a member of at least i subclass in the specialization. For example, if every EMPLOYEE must be either an HOURLY_EMPLOYEE or a SALARIED_EMPLOYEE, then the specialization {HOURLY_EMPLOYEE, SALARIED_EMPLOYEE} in Figure 8.i is a total specialization of EMPLOYEE. This is shown in EER diagrams past using a double line to connect the superclass to the circle.
A single line is used to display a partial specialization, which allows an entity not to belong to any of the subclasses. For example, if some EMPLOYEE entities do not belong to any of the subclasses {SECRETARY, ENGINEER, TECHNICIAN} in Figures 8.1 and 8.4, and so that specialization is partial. Discover that the disjointness and abyss constraints are independent.
Hence,nosotros accept the following four possible constraints on specialization:
? Disjoint, full
? Disjoint, partial
? Overlapping, total
? Overlapping, partial
Of course, the correct constraint is determined from the existent-world meaning that applies to each specialization. In full general, a superclass that was identified through the generalization process usually is total, considering the superclass is derived from the subclasses and hence contains only the entities that are in the subclasses. Certain insertion and deletion rules apply to specialization (and generalization) equally a issue of the constraints specified before.
Some of these rules are every bit follows:
? Deleting an entity from a superclass implies that it is automatically deleted from all the subclasses to which it belongs.
? Inserting an entity in a superclass implies that the entity is mandatorily inserted in all predicate-defined (or aspect-defined) subclasses for which the entity satisfies the defining predicate.
? Inserting an entity in a superclass of a full specialization implies that the entity is mandatorily inserted in at to the lowest degree one of the subclasses of the specialization.
Oft Asked Questions
+
Ans: Specialization is the procedure of defining a ready of subclasses of an entity type; this entity type is called the superclass of the specialization. Nosotros use the term generalization to refer to the process of defining a generalized entity type from the given entity types. view more..
+
Ans: Databases are widely used in enterprises, banking and finance, universities, airlines, telecommunication, etc. view more..
+
Ans: A database-direction organization (DBMS) is a drove of interrelated data and a set of programs to admission those data. view more..
+
Ans: we discuss differences betwixt specialization/generalization lattices (multiple inheritance) and hierarchies (single inheritance), and elaborate on the differences between the specialization and generalization processes during conceptual database schema pattern. Nosotros discuss constraints that apply to a single specialization or a single generalization. view more than..
+
Ans: it is sometimes necessary to correspond a single superclass/bracket relationship with more one superclass, where the superclasses represent dissimilar entity types. In this example, the subclass will represent a collection of objects that is a subset of the Marriage of distinct entity types. view more than..
+
Ans: For our sample database application, consider a UNIVERSITY database that keeps runway of students and their majors, transcripts, and registration as well as of the academy'due south form offerings. The database also keeps rails of the sponsored research projects of faculty and graduate students. view more..
+
Ans: The basic notation for specialization/generalization is to connect the subclasses past 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..
+
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 noesis by creating an ontology that describes the concepts of the domain and how these concepts are interrelated. view more..
+
Ans: Recovery from transaction failures normally means that the database is restored to the nearly contempo consistent state just before the time of failure. To practice this, the organisation must keep information near the changes that were applied to data items past the various transactions. view more than..
+
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 also restrictive for database concurrency control purposes, and and so are not used in practise. Then shared/sectional locks - also known equally read/write locks - which provide more than general locking capabilities and are used in practical database locking systems. view more than..
+
Ans: In this department nosotros discuss the concepts of concurrent execution of transactions and recovery from transaction failures. view more than..
+
Ans: Conceptual modeling is a very of import phase in designing a successful database awarding. More often than not, the term database awarding refers to a particular database and the associated programs that implement the database queries and updates. view more..
+
Ans: Data is converted into data, and information is and then evaluated and organised so that it can be used purposefully every bit noesis. view more..
+
Ans: A database organization is a collection of interrelated data and a fix of programs that let users to access and alter these information. A major purpose of a database system is to provide users with an abstract view of the information. That is, the system hides certain details of how the data are stored and maintained. view more..
+
Ans: In this section we discuss the concepts of concurrent execution of transactions and recovery from transaction failures view more..
+
Ans: This chapter discusses techniques for securing databases against a diverseness of threats. Information technology also presents schemes of providing access privileges to authorized users. view more than..
+
Ans: A database system provides a data-definition language to specify the database schema and a data-manipulation language to express database queries and updates. view more..
+
Ans: Database design mainly involves the design of the database schema. The blueprint of a complete database application environs that meets the needs of the enterprise being modelled requires attention to a broader set of problems. view more..
Source: https://padakuu.com/constraints-and-characteristics-of-specialization-and-generalization-hierarchies-253-article
0 Response to "The Completeness Constraint Can Be Partial or Total."
Post a Comment