General Principles for Relationships (original) (raw)

Developing High Quality Data Models, 2011

Abstract

This chapter illustrates some examples of traps found with relationship types in data models. It then demonstrates how the principles for conceptual, integration, and enterprise data models can help overcome or avoid these issues. The principles for relationship types are—activities should be represented by entity types (not relationship types), relationship types (in the entity/relationship sense) should only be used to represent things about which there is nothing to say, and cardinality constraints on relationship types should be true always. Applying the principles makes the data models more consistent, and they are more likely to support the data needed, rather than just the data first thought of. Making the data model more general is relatively easy. One simply removes the constraints that may not always be true. Introducing the fudge data to overcome the incorrect cardinalities can have expensive consequences. Sometimes cardinalities are set to one-to-many, meaning one at a time, when the cardinalities are really many-to-many over time because the relationship type is transferable. Imposing restrictions through the data structure means—arbitrary or inappropriate restrictions are placed on the data that can be held, historical data about a relationship cannot be held, the entity type will only work within the context defined, and the resultant system is harder to share.

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