LOGICAL MODELING

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When reviewing these problems with the objective of trying to formulate potential solutions, the following are the main requirements that need to be satisfied:

Accurate reporting of the facts. It is very important that, in a dimensional model, whenever a fact entry is joined to a dimension entry, the fact must join to the correct dimension with respect to time. Where a snowflake schema exists, the fact entry must join to the correct dimension entry no matter where that dimensional entry appears in the hierarchy. In reality, it is recognized that this can never be wholly achieved in all cases, as deficiencies in the capture of data from the operational systems of the organization will impinge on our ability to satisfy this requirement. We can hope only to entirely satisfy this requirement insofar as the warehouse model itself is concerned and to minimize, as far as possible, the negative effect caused by the operational systems.

Accurate recording of the changes in entities to support queries involving customer circumstances and dimension browsing. Queries such as these represent a very significant component of the data warehouse usage. It is important to ensure that the periods of existence (validity) of dimensions, relationships, and attributes are recorded accurately with respect to time where this has been identified as a business requirement. Again, the ability to do this is constrained by the accuracy and quality of data supplied by the operational systems. It is important that the warehouse does not compound the problem.

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