ean 128 excel 2013 Schemas in Software

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External Schema A user who is interested in obtaining information from the database may need only an external description of the content of the database The user can obtain a list of the entities, their attributes, and the type characteristics of these attributes from the system The user can request the data in interesting combinations and specify data reduction processes The external schema for a user of the database may be closely related to the view model which was used to construct this portion of the database model Many external schemas are composed of subsets of the database relations and subsets of the attributes of the selected relations Where the database submodel di ers structurally from the implemented database, substantial transformations may be required, including joins and sorts Simple transformations include the speci cation of record segments Substantial delays may be encountered if the transformations are major A user who uses the database frequently will, of course, form concepts about the system by learning what the system does poorly and what it does well and willadapt to the database, ignoring the original view External schemas may be changed to adapt to the needs of the user without impacting other schemas An example of simple subschema de nition is shown in Example 8-11 A VIEW on the relations or TABLEs in IBM SQL/DS permits projection and rearranging of attribute columns, selection of rows, and speci cation of derived variables The VIEWs in SQL/DS never create relations; all mapping is performed at the time when the view is used The SQL language is summarized in Sec 9-2
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Example 8-11
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External Subschema De nition
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Given a relation Children with data in traditional measurements: Children: RELATION child id, guardian, class, rank, age, height, weight, year 1; we de ne a view for the modern metric-thinking school nurse
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CREATE VIEW School nurse weight kg ) ( child, age, height cm, AS SELECT child id, age, height * 254, weight * 04536 FROM Children; A database will be accessed via multiple external schemas One external schema contains the entries required for the data processing of one area of interest External schemas may overlap to re ect overlapping data models The use of multiple schemas also aids in the maintenance of the integrity of the database, since fewer of the relations are exposed to processes which may contain errors This concept of an external schema to constrain access provides control at little additional cost If access programs are not constrained, a database system, in order to protect data from unauthorizee access, will have to verify of every record and eld address submitted as being appropriate for the user In SQL the original database TABLE creator can de ne VIEWS and assign them to speci c users with certain privileges to provide access control Multiple active processes may use the same external schema Distinct external schemas may overlap If any overlapping schemas are used at the same time, it is desirable that the processes share the schema entries at some level This provides a linkage which the system can use to avoid problems of access interference Only
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privilege information, tied to the users rather than to the database elements, must remain separate
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Conceptual Schema The external view is derived from the overall conceptual schema which represents the entire database model Here all relationships are described The conceptual model covers the information-processing needs of an enterprise or a large portion of the enterprise As the real world changes, the conceptual schema will have to be adjusted It is desirable that derived external schemas using conceptual elements which are changed can be restated to provide a relatively constant user interface Only the external schema which required a conceptual change due to a related change in its view model will require modi cation If all users access the database through external schemas, the conceptual model may not be physically present during database processing; its main function is design and schema generation Internal Schema The operational management of the stored les requires further information to be placed in an internal schema The internal schema de nes where the database attribute values are placed and how they are accessed The decisions encoded in the internal schema present the aggregate requirements of all the users Here the notion of a database administrator appears The load estimates or measurements, as de ned in Sec 5-4, are applied here and are balanced with response-time requirements for speci c transactions Figure 8-10 provided examples of the tools available to tune the performance of a database The principal concept is always the addition of redundant access structures and the maintenance of locality Control over actual and potential data derivations is part of the internal schema To provide input to performance control functions, the internal schema may also contain descriptive statistical data elements which are updated during operation The TOD system, for instance, collects in its schema the access counts to attributes, to be used for periodic restructuring by the data administrator If the database is distributed over multiple physical locations, corresponding multiple internal schemas can be used to implement the single conceptual schema
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8-5-2 Physical Organization of Schema Characteristics A database schema may be organized by attribute or by characteristic: names, domains, titles, control of access to data, etc Certain database processes will not need all the characteristics that we have associated with each data element Processing programs speci cally may only need access to the type and length characteristics of the data elements If data are only to be moved, only the length and count are needed The descriptive information which contains titles, unit speci cations, etc, can remain inaccessible This physical organization reduces memory and paging requirements The TOD schema table, for instance, is transposed during translation to provide the schema tables in a compact form to the interpreter This means that, for instance, all TITLE information is available as a single array to the output processor, and all TYPE information is placed into another array to be used by the computational processes
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