CHAPTER 2 Database Environment

Chapter Objectives (Chapter goal)

In this chapter you'll learn: (in this chapter, you'll learn:)

• The purpose and origin of the Three-level database architecture.   (three-tier database architecture and origins.
• the contents of the external, conceptual, and internal levels. (External, conceptual, internal-level content.)
• The purpose of the external/conceptual and the conceptual/internal mappings. (External/conceptual, and conceptual/internal mapping purposes.)
• The meaning of logical and physical data independence. (The meaning of logical and physical data independence.)
• The distinction between a data definition Language (DDL) and a data manipulation Language (DML).   (Data definition language The difference between (DDL) and Data Manipulation language (DML). )
• A Classification of data models.   (the classification of the database model.
• The purpose and importance of conceptual modeling.   (Purpose and importance of conceptual modeling)
• The typical functions and Ser Vices that a DBMS should provide.   (the typical features and services that the DBMS should provide.)
• the function and importance of the system catalog.   (function and importance of the systems directory)

A major aim of a database system is to provide users with an abstract view of data, hiding certain details of what data is Stored and manipulated. Therefore, the starting point for the design of a database must is an abstract and general description of the information Requirements of the Organization, that is, was represented in the database. In this chapter, and throughout of the term "organization" loosely to mean the whole organization or part O f the organization. For example, with the Dreamhome case study, we are interested in modeling:

• The "Real=-world" entities staff, propertyforrent, Privateowner, and Client;
• attributes describing properties or qualities of each entity (for example, each staff entry have a name, position, and salary);
• relationships between these entities (for example, Staff manages propertyforrent).

Furthermore, because a database is a shared resource, each user may require a different view of the data held in the Datab Ase. To satisfy these needs, the architecture of most commercial DBMSs available today are based to some extent on the so-called ANSI-SPARC architecture. In this chapter, we discuss various architectural and functional characteristics of DBMSs.

Structure of this chapter        under section 2.1, we examine the Three-level ansi-sp ARC architecture and its associated benefits. In sections 2.2, we consider the types of language that is used by DBMSs, and in section 2.3, we introduce the concepts of Data models and conceptual modeling, which we expand on later parts of the book. In section 2.4, we discuss the functions, and we would expect a DBMS to provide. The examples in this chapter is drawn from the Dreamhome case study, which we discuss more fully in sections 11.4 and AppE Ndix a. 

Much of the material in this chapter provides important background information on DBMSs. However, the reader who's new to the area of the database systems could find some of the material difficult to comprehend fully On first reading. Does not is too concerned about this, but is prepared to revisit parts of this chapter at a later data when you have read Su Bsequent chapters of the book.

2.1 The Three-level Ansi-sparc Architecture

An early for a standard terminology and general architecture for database systems is produced in 1971 by the DBTG appoint Ed by CODASYL in 1971. The DBTG recognized the need for a two-level approach with a system view called the schema and user views called Subschem As.

CHAPTER 2 Database Environment