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Database Systems: Concepts, Design and Applications by S. K. Singh

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6.4. Problems with E-R Models

Some problems, called connection traps, may arise when creating an E-R model. The connection traps normally occur due to a misinterpretation of the meaning of certain relationships. There are mainly two types of connection traps:

6.4.1. Fan Traps

In a fan trap, a model represents a relationship between entity types, but the pathway between certain entity occurrences is ambiguous. A fan trap may exist where two or more one-to-many (1:N) relationships fan out from the same entity. Fig. 6.19 (a) shows an example of fan trap problem.

Fig. 6.19. Example of fan trap and its removal

(a) An example of fan trap

(b) Semantic net of E-R model

(c) Restructured E-R model to eliminate fan trap

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