police excel ean 128 Hierarchical Databases in Software

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In the development of view models hierarchical concepts as nests play an important role A model based on hierarchies can be directly implemented using nest-oriented le structures and appropriate schema facilities Such a model is then restricted to one entity relation and its nest relations In order to accommodate data that do not t within a single tree, a hierarchical database system will allow the existence of multiple trees or a forest The trees of the forest will have di erent heights; there are often many single-level trees, equivalent to entity relations in rst normal form These contain data structures which do not t into the hierarchical structure A small forest based on the example relations used in Chap 7 is shown in Fig 9-5, the structure is again visually simpli ed through the use of Bachman arrows
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A database model which is bound to any prede ned structure will lose considerably in exibility On the other hand, the de nition of access paths, implied by the structure, means that these paths do not have to be created during query or update processing, but that they already exist when needed The advantage of such early binding is often a considerable gain in processing speed and a simpli cation of query formulation when the data model relevant to the query is within the bound database structure Database structures can be implemented to match almost any model but will never be able to match all models satisfactorily One person s hierarchy is often another one s bureaucracy The hierarchical model is relatively simple and satis es the conceptual needs of database users in many cases 9-4-1 Manipulating Trees A tree is by de nition not in rst normal form We are dealing therefore with a basic structure whose elements cannot be described by the triple: relation name, tuple name, attribute name Relational operations, such as projection or join, are di cult to de ne and rarely implemented in these systems It is, however, possible in most cases to reformulate relational queries into search strategies on trees, or to transform unambiguous hierarchical queries into equivalent relational queries Relations which will often participate in the equivalent of joins are best kept at the top level of the hierarchy; this includes primary and referenced entity relations as well as lexicons Tuples in the lower-level nests of a tree provide only record segments, since ancestor data are a logical part of such tuples
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If a segment in a hierarchical structure has been selected, for instance, employee="Hare" in Fig 9-6, we can identify with this segment the structural sequence of all segments which are its owners or ancestors, namely, (department="Assembly", file="Personnel"), as well as the subtrees for this segment, namely, nests of (children, education, and supervision) This construct has been named a broom, and the segments which make up the broom of employee="Hare" are indicated in the gure A broom is often the unit to be manipulated here, rather than a single segment Segments in hierarchical structures depend greatly on their ancestors and hence cannot be freely moved about The operations union, intersection, and complement of two brooms remain de nable in terms of the operations , , and on their members
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Operations with brooms require some care The broom based on "Hare" includes "Assembly", as does a broom based on "Gander" The union of the two brooms includes all elements in both brooms The intersection of the two brooms contains only "Assembly" and "Personnel" The complement of the broom of "Hare" contains all employees except "Hare", but not "Assembly" or "Personnel" The results of intersection and complement are hence incomplete brooms, and subsequent operations may not be obvious, as will be shown in Example 9-14 Retrieval of records based on selected segments requires de nition of the record based on a qualifying segment To identify a segment in a nest qualifying terms as FIRST, LAST, NUMBER = ns, ANY, ALL can be used To nd now the current school for the oldest child, we can request LASTchildrenLASTeducation or to nd who sent all his children to St Mary s, ALLchildrenANYeducation = "St Mary" An understanding of the structure plays an important role in these systems A search for a speci c record is constrained by the nesting structure In order to simplify the construction of queries, we introduce the operation BROOM which de nes all segments of the broom for a segment Membership of a
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