Chapter Objectives （章节目标）
In this chapter you will learn: （在本章中，你将学习：）
- The purpose and origin of the three-level database architecture. （三层数据库的架构和起源。）
- The contents of the external, conceptual, and internal levels. （外部、概念、内部层次的内容。）
- The purpose of the external/conceptual and the conceptual/internal mappings. （外部/概念、和概念/内部映射的目的。）
- The meaning of logical and physical data independence. （逻辑和物理数据独立性的意义。）
- The distinction between a Data Definition Language (DDL) and a Data Manipulation Language (DML). （数据定义语言（DDL）和数据操作语言（DML）之间的区别。）
- A classification of data models. （数据模型的分类。）
- The purpose and importance of conceptual modeling. （概念建模的目的和重要性）
- The typical functions and services that a DBMS should provide. （DBMS 应提供的典型功能和服务。 ）
- The function and importance of the system catalog. （系统目录的功能和重要性）
A major aim of a database system is to provide users with an abstract view of data, hiding certain details of how data is stored and manipulated. Therefore, the starting point for the design of a database must be an abstract and general description of the information requirements of the organization that is to be represented in the database. In this chapter, and throughout this book, we use the term "organization" loosely to mean the whole organization or part of the organization. For example, in the DreamHome case study, we may be 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 has 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 database. To satisfy these needs, the architecture of most commercial DBMSs available today is 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 In Section 2.1, we examine the three-level ANSI-SPARC architecture and its associated benefits. In Section 2.2, we consider the types of language that are used by DBMSs, and in Section 2.3, we introduce the concepts of data models and conceptual modeling, which we expand on in later parts of the book. In Section 2.4, we discuss the functions that we would expect a DBMS to provide. The examples in this chapter are drawn from the DreamHome case study, which we discuss more fully in Section 11.4 and Appendix A.
Much of the material in this chapter provides important background information on DBMSs. However, the reader who is new to the area of database systems may find some of the material difficult to comprehend fully on first reading. Do not be too concerned about this, but be prepared to revisit parts of this chapter at a later data when you have read subsequent chapters of the book.
2.1 The Three-level ANSI-SPARC Architecture
An early for a standard terminology and general architecture for database systems was produced in 1971 by the DBTG appointed 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 subschemas.