By David Lefkovitz Ph.D (auth.)

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Additional resources for File Structures for On-Line Systems

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Examples are NAME/ SMITH, AGE/21. Within the record, both keys and qualifiers are stored in the general form Key Name/Value or Qualifier Name/ Value. * In the case of the keys, however, a list or file partition is created for each Key Name/Value pair. list approach, t and the other is the inverted list approach. In the former, a Link Address is required within the record in order to form the list within the file; hence, in addition to the Key Name/Value, a Link Address, which indicates the next record in the DASD that also contains the identical Key Name/Value (or Value), is also stored in the key field.

In addition to arithmetic value comparisons, some systems may require functional value comparisons, which add both power and complexity that are warranted only in very special circumstances. An example is to retrieve from a file all ships that are less than the average ship tonnage. " Thus, the user can specify an arithmetic value for comparison that is the result of a computation instead of being a prestated constant. How this is accomplished in real-time retrieval is discussed in Chapter IV. The decision to accept or reject a rec~rd based on arithmetic value comparison with a qualifier is made wholly within the record itself and is not dependent or contingent on values in or processes performed on other records either in the file or in the list of retrieved records (from the DASD).

In summary, there is a logical relationship between certain records of this file, which is established when the file is originally constructed. For example, the record labeled A has a specific inferior relationship in the hierarchy to the record labeled C, and this in turn has an inferior relationship to record G. In contrast, an Associative relationship, as indicated in the diagram by broken lines, is one in which records are related by virtue of the fact that each contains a specific key that is identical.

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