Chapter 18: Different Relational Models

Chapter 18: Different Relational Models

Overview

There is no such thing as the relational model for databases anymore than there is just one geometry.

In the 19th century, Bolyai, Gauss, and Lobachevski all realized that the parallel postulate of Euclidean geometry could be changed and whole new geometries developed. For example, if I draw a triangle on a plane, it always has 180 degrees; if I draw it on the surface of a sphere, it always has more than 180 degrees; and if I draw it on the surface of a trumpet, it always has less than 180 degrees.

Which geometry is true? Well, it depends where I am. My backyard is roughly a plane, the surface of the earth is roughly a sphere, and the gravity well of a star is roughly a trumpet.

Models are consistent and constructible. Consistent means that you cannot prove something to be both true and false using the rules of the model. Constructible means that I can build a finite version of it in the real world.

18.1 Chris Date = No Duplicates, No NULLs

Although Chris Date was not the first person to work with the relational model, his version is the simplest and closest to the ?usual file model? of data, so it is where we will start this chapter.

Date?s relational model allows no duplicate rows in tables and has no NULL . Missing values are shown either by special values in the encoding schemes or by an indicator...

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Chapter 18: Different Relational Models

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