TABLE OF CONTENTS Previous chapters discussed how to model the structure of an application in terms of a conceptual schema, and then map it to a logical schema for implementation in a relational database system. Although much of the design and mapping can be automated, humans are required to perform CSDP step 1, since verbalizing the relevant facts about the real world involves human understanding. Given the informal nature of this initial step in modeling the UoD, it is not surprising that people often come up with different ways of describing the same reality.
Hence the same application may be modeled by more than one conceptual schema. Moreover, the same conceptual schema may be mapped in different ways to a logical schema, either by choosing a different type of logical model (e.g., object-relational instead of pure relational) or by overriding the default mapping choices (e.g., choosing separate subtype tables or denormalizing). Finally the same logical schema may be implemented in more than one way in the physical schema (e.g., different index choices).
To help you choose from such a variety of different models, this chapter examines various ways in which schemas may be transformed to provide alternatives, then presents guidelines for optimizing your choice. On the way, we ll cover some theory on schema transformations and normalization, to ensure a proper understanding of the underlying concepts. We ll start at the conceptual level, then move down to the logical and physical levels. An example will then illustrate how the...
