If we look at a data model, is it just a tangle of entities and relationships, or is there a structure that is not immediately apparent? Of course, we know that different entities can be grouped together because of relationships between them or because they participate in the same business processes. Even so, is there a structure within databases in general that is independent of a particular business domain and that does not rely on relationships between entities? This article argues that such a structure does exist, and that any database can be viewed as a successive series of layers of different kinds of data. Each layer has its own particular properties and constraints for the layers below it.

Before describing this general structure, it is worth questioning whether attempting to categorize entities in a data model is just an academic exercise, or if it has any real significance for data administrators and other IT professionals. Categorization can be helpful if it allows us to target specific data administration tasks only to those entities for which these tasks are relevant. No data administrator wants to blindly execute the same tasks for every entity in a data model if this is not necessary. In addition, there are other IT professionals who work with database designs and physically implemented databases who may also be able to benefit from an approach that lets them distinguish between different classes of entities or tables.

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