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It is tempting to normalize relations based on an analysis of the values of their attributes It should be realized, however, that values in the database at a given moment in time may show relationships which are accidental and not based on true functional dependencies
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An observation made from Fig 7-6, All employees born in 1938 are "Welders" , should not be taken to be a functional dependency
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On the other hand, the main reason for the existence of databases is to support the search for new dependencies If we nd that all or many welders develop a certain type of disease, and we can eventually prevent this dependency, then the e ort to develop and collect data for this database will have been very worthwhile Such an analysis is the concern of information-retrieval processes and not part of the view model
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Figure 7-18
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An erector set of relation types
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We have in this section discussed how semantic concepts regarding one particular view model or eventual database model can be described in terms of ve relatively simple relation types and their subrelations: Entity relations Referenced entity relations Nest relations Associative relations Lexicons and three connection types Reference connections Ownership connections Subset connections The two concepts are duals of each other; a relation type de nes the connections it participates in and a connection type de nes the relation types it connects Both concepts are useful, however, in dealing with models and their implementation, and both are found instantiated in database systems Constraints between relations are more easily described using connections Data elements and their semantics are better described using relations A data-processing transaction is de ned on both Figure 7-18 provides a graphical image of these types using the Bachman arrows for ownership connections, arrows with tails for reference connections, and plain arrows for functional dependencies within the tuples themselves Use of these graphic symbols is extremely useful in presenting view models for review to the participants in a database design process No tables or lists will illustrate the dependencies and hence the semantics of the view model as well When the view is approved, automatic processing of the information in the view models to arrive at a database model becomes feasible We will review the sematics of the relation types below, using the rules de ned with the connections A A 1 2 3 primary entity relation: Not referenced within the view model The ruling part de nes the entity The existence of tuples is determined externally, ie, by the user
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B A referenced entity relation: 1 Referenced from within the view model 2 The ruling part de nes the entity and establishes domains for referencing attributes A ruling part typically has a single attribute 3 Existence of tuples is determined externally, but deletion is constrained by existing references C A nest relation: 1 Each tuple must have an owner tuple within the view model 2 The ruling part de nes one speci c owner and the tuple within the owned set 3 An owned nest can have zero or more tuples
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Database Structure
D An associative relation of order n: 1 Each tuple has n owners within the view model 2 The ruling part de nes each of n speci c owners only 3 One combination of owners can have 0 or 1 tuple in the association E A lexicon: 1 Referenced within the view model 2 Either part can be the ruling part 3 The existence of tuples is determined externally but deletion is constrained by existing references 4 Its existence is transparent to the model F Subrelations: 1 Referenced from any general relation 2 The ruling part matches the ruling part of the general relation 3 The dependent part contains attributes which do not apply to non matching tuples within the general relation 4 Insertion requires existence of a matching tuple in the general relation The construction of each of these relation types obeys certain rules so that we can also describe these types using a Backus-Naur notation Since such a description is essentially syntactic, some semantic constraints are added to the rules in quotes
Table 7-4
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