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1 Introduction
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last, the account may contain either $450 or $400, rather than the correct value of $350 To guard against this possibility, the system must maintain some form of supervision But supervision is dif cult to provide because data may be accessed by many different application programs that have not been coordinated previously Security problems Not every user of the database system should be able to access all the data For example, in a banking system, payroll personnel need to see only that part of the database that has information about the various bank employees They do not need access to information about customer accounts But, since application programs are added to the system in an ad hoc manner, enforcing such security constraints is dif cult These dif culties, among others, prompted the development of database systems In what follows, we shall see the concepts and algorithms that enable database systems to solve the problems with le-processing systems In most of this book, we use a bank enterprise as a running example of a typical data-processing application found in a corporation
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A database system is a collection of interrelated les and a set of programs that allow users to access and modify these les 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
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For the system to be usable, it must retrieve data ef ciently The need for ef ciency has led designers to use complex data structures to represent data in the database Since many database-systems users are not computer trained, developers hide the complexity from users through several levels of abstraction, to simplify users interactions with the system: Physical level The lowest level of abstraction describes how the data are actually stored The physical level describes complex low-level data structures in detail Logical level The next-higher level of abstraction describes what data are stored in the database, and what relationships exist among those data The logical level thus describes the entire database in terms of a small number of relatively simple structures Although implementation of the simple structures at the logical level may involve complex physical-level structures, the user of the logical level does not need to be aware of this complexity Database administrators, who must decide what information to keep in the database, use the logical level of abstraction
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
1 Introduction
Text
The McGraw Hill Companies, 2001
1
Introduction
View level The highest level of abstraction describes only part of the entire database Even though the logical level uses simpler structures, complexity remains because of the variety of information stored in a large database Many users of the database system do not need all this information; instead, they need to access only a part of the database The view level of abstraction exists to simplify their interaction with the system The system may provide many views for the same database Figure 11 shows the relationship among the three levels of abstraction An analogy to the concept of data types in programming languages may clarify the distinction among levels of abstraction Most high-level programming languages support the notion of a record type For example, in a Pascal-like language, we may declare a record as follows: type customer = record customer-id : string; customer-name : string; customer-street : string; customer-city : string; end; This code de nes a new record type called customer with four elds Each eld has a name and a type associated with it A banking enterprise may have several such record types, including account, with elds account-number and balance employee, with elds employee-name and salary At the physical level, a customer, account, or employee record can be described as a block of consecutive storage locations (for example, words or bytes) The language
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