The notion of time pervades every corner of the data warehouse. Most of the fundamental measurements we store in our fact tables are time series, which we carefully annotate with time stamps and foreign keys connecting to calendar date dimensions. But the effects of time are not isolated just to these activity-based time stamps. All of the other dimensions that connect to fact tables, including fundamental entities such as Customer, Product, Service, Terms, Location and Employee, are also affected by the passage of time. As data warehouse managers, we are routinely confronted with revised descriptions of these entities. Sometimes the revised description merely corrects an error in the data. But many times the revised description represents a true change at a point in time of the description of a particular dimension member, such as Customer or Product. Because these changes arrive unexpectedly, sporadically and far less frequently than fact table measurements, we call this topic slowly changing dimensions (SCDs). In this months column and the next, Ill show how to implement the three basic types of SCDs.
The Three Types
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