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Designing a Data Warehouse: Supporting Customer Relationship Management by Chris Todman

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THE IMPLEMENTATION OF RETROSPECTION

Introduction

We begin this section with a general rule:

Every query executed in a data warehouse must have a time constraint.

If an executed query does not have an explicit time constraint, then the inferred time period is “for all time.” Queries that embrace all of time, insofar as the data warehouse is concerned, can be generally regarded as nonsensical because “for all time” simply means the arbitrary length of time that the database has been in existence. Whereas it may be sensible to aggregate across all customers or all products in order to ascertain some information about, say, total revenue for a period of time, it does not make sense to apply the same approach to time under normal circumstances. The ...

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