police code ean 128 excel De ning the Elements for a Database in Software

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De ning the Elements for a Database
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existing procedures must be studied in order to obtain a composite picture of the actions that are carried out Documentation of data sources and destinations is collected The frequency of use, the desired response time, and even the accuracy requirements are captured if feasible Similar information is obtained for new procedures and the data elements required for their operation Here estimates may be needed, but documentation of assumptions made is at least as important to the design process as the documentation of established processes
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Figure 8-1 sketches an example of a very simple data ow In practice, tabular descriptions for each variable are more convenient than a owchart The equivalent ow would be described as as follows:
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Example 8-1 Variable
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Table of Data Elements
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Source Destination
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Elements may, of course, also have multiple destinations The analysis of current procedures has to be carried out with due regard to the actual information ow The fact that certain reports produced contain a speci c data element is not necessarily an indication that the element is being used, or if it
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Schemas
is used, that it is the best means of presenting the desired information On the other hand, there often are informal methods of data distribution which are not obvious during a system analysis which depends wholly on existing documents Talking is an important, unformalized, and exible form of communication
For example, the fact that an engineering department manager warns an inventory clerk that he expects new employees so that necessary tools will be available when they arrive is an information function easily overlooked when automating a personnel system
8-1-2 Characteristics of Data We begin by describing the characteristics of data elements The most important description of a data element are its name and its domain These are familiar concepts to any programmer
Programming languages have always provided facilities to name and characterize data elements The domain of a data element, needed for computation, is mostly given as the type and perhaps the length Some modern languages permit further speci cations Other characteristics, for instance, the largest and smallest value expected, are de ned implicitly or in the program documentation For the database complete and explicit descriptions are advantageous
We will present the quantitative characteristics of data elements type or domain, length, conditionals, count in this subsection Qualitative speci cations of data element characteristics are presented in Sec 8-1-3 The number of attributes used to describe each type of data element is su ciently large that we may keep the schema itself as a relation Names have been used throughout to de ne attributes in records and relations The name given to a data element by a programming language is restricted by fairly simple rules These rules generally allow a short, variable length string of alphabetic and numeric characters; the rst character has to be alphabetic This string is easily parsed or separated from other elements that make up the statements accepted by the schema language Compatibility between the programming language and the schema language is desirable If multiple languages are used, the sum of compatibilities can restrict names unduly Names used in les and databases are global; ie, they are bound to their meaning over all programs while the les or database is open The schema will be used by many programs over a long time In programming languages a name can have a di erent de nition as the process changes scope In a database the scope of a name of a data element is a ected only by structural scope, the name is de ned within the database or relation that contains the data element To locate the values, the position or byte number within the record is kept associated with the names in the schema If the eld allocation is exible, an approach as seen in a pile le may be used A coded name is kept with each element in the record The schema keeps the codes with the names to allow translation from name to code and vice versa Most schema language translators will assign positions only according to their own data allocation methods Some languages permit the users to specify the position explicitly so that the schema can be applied to previously existing les
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