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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, some of which are still in use today.

The CASCADE rule presents a similar problem, as shown in Figure 11-7. This figure contains exactly the same data as in Figure 11-6, but all three delete rules have been changed to CASCADE. Suppose you try to delete Bob from the BOYS table. The delete rules force the DBMS to delete Rover (who likes Bob) from the PETS table, which forces you to delete Betty (who likes Rover) from the GIRLS table, which forces you to delete Sam (who likes Betty), and so on, until all of the rows in all three tables have been deleted. For these small tables this might be practical, but for a production database with thousands of rows, it would quickly become impossible to keep track of the cascaded deletions and retain the integrity of the database. For this reason, DB2 enforces a rule that prevents referential cycles of two or more tables where all of the delete rules are CASCADE. At least one relationship in the cycle must have a RESTRICT or SET NULL delete rule to break the cycle of cascaded deletions.

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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 as part of the computer s operating system, kept track of the names and locations of the files. File management systems are still widely used today you are probably familiar with the files-and-folders structure provided by the file system on Microsoft Windows or Apple s Macintosh operating systems. Similar file systems are used by UNIX-based servers and all commercial computer systems. A file management system basically has no data model; it knows nothing about the internal contents of files. At best, the file system might maintain file type information

Figure 11-7.

Part I:

Foreign Keys and NULL Values *

along with the filename, allowing it to distinguish between a word processing document and a file containing payroll data But knowledge about the internal contents of a file what individual pieces of data it contains and how that data is organized is embedded in the application programs that use the file, as shown in Figure 4-1 In this payroll application, each of the COBOL programs that processes the employee master file contains a file description (FD) that describes the layout of the data in the file If the structure of the data changes for example, if an additional item of data is to be stored for each employee every program that accesses the file has to be modified This isn t a problem for files containing word processing documents or spreadsheets, which are usually processed by a single program.

CHECK (VALUE BETWEEN 11 AND 99) then the OFFICES table definition could be modified to: Define the OFFICES table and its columns. CREATE TABLE (OFFICE CITY REGION MGR TARGET SALES OFFICES VALID_OFFICE_ID NOT NULL, VARCHAR(15) NOT NULL, VARCHAR(10) NOT NULL, INTEGER, MONEY, MONEY NOT NULL)

But in corporate data processing, files are often shared among dozens or even hundreds of programs, as in Figure 4-1 As the number of files and programs grows over time, more and more of a data-processing department s effort goes 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: move 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 Moreover, DBMSs are an extension of file management systems rather than a replacement for them.

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