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AN OVERVIEW OF SQL
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This chapter described the development of SQL and its role as a standard language for relational database management: I SQL was originally developed by IBM researchers, and IBM s strong support of SQL is a key reason for its success. I There are official ANSI/ISO SQL standards and several other SQL standards, each slightly different from the ANSI/ISO standards. I Despite the existence of standards, there are many small variations among commercial SQL dialects; no two SQLs are exactly the same. I SQL has become the standard database management language across a broad range of computer systems and applications areas, including mainframes, workstations, personal computers, OLTP systems, client/server systems, data warehousing, and the Internet.
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Relational Databases
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SQL: The Complete Reference
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atabase management systems organize and structure data so that it can be retrieved and manipulated by users and application programs. The data structures and access techniques provided by a particular DBMS are called its data model. A data model determines both the personality of a DBMS and the applications for which it is particularly well suited. SQL is a database language for relational databases that uses the relational data model. What exactly is a relational database How is data stored in a relational database How do relational databases compare to earlier technologies, such as hierarchical and network databases What are the advantages and disadvantages of the relational model This chapter describes the relational data model supported by SQL and compares it to earlier strategies for database organization.
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As database management became popular during the 1970s and 1980s, a handful of popular data models emerged. Each of these early data models had advantages and disadvantages that played key roles in the development of the relational data model. In many ways, the relational data model represented an attempt to streamline and simplify the earlier data models. To understand the role and contribution of SQL and the relational model, it is useful to briefly examine some data models that preceded the development of SQL.
File Management Systems
Before the introduction of database management systems, all data permanently stored on a computer system, such as payroll and accounting records, was stored in individual files. A file management system, usually provided by the computer manufacturer as part of the computer s operating system, kept track of the names and locations of the files. The file management system basically had no data model; it knew nothing about the internal contents of files. To the file management system, a file containing a word processing document and a file containing payroll data appeared the same. Knowledge about the contents of a file which data it contained and how the data was organized was embedded in the application programs that used the file, as shown in Figure 4-1. In this payroll application, each of the COBOL programs that processed the employee master file contained a file description (FD) that described the layout of the data in the file. If the structure of the data changed for example, if an additional item of data was to be stored for each employee every program that accessed the file had to be modified. As the number of files and programs grew over time, more and more of a data-processing department s effort went into maintaining existing applications rather than developing new ones. The problems of maintaining large file-based systems led in the late 1960s to the development of database management systems. The idea behind these systems was simple:
4:
Relational Databases
AN OVERVIEW OF SQL
Figure 4-1.
A payroll application using a file management system
take the definition of a file s content and structure out of the individual programs, and store it, together with the data, in a database. Using the information in the database, the DBMS that controlled it could take a much more active role in managing both the data and changes to the database structure.
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