SQL and Relational Theory, 2nd Edition

SOME DATABASE PRINCIPLES

In Chapter 1, I said I was interested in principles, not products, and we’ve encountered several principles at various points in the book. Here I collect them together for ease of reference.

• The Information Principle (also known as The Principle of Uniform Representation or The Principle of Uniformity of Representation): The database contains nothing but relvars; equivalently, the entire information content of the database at any given time is represented in one and only one way—namely, as explicit values in attribute positions in tuples in relations.^[184]

• The Closed World Assumption: Let relation r correspond to predicate P. If tuple t appears in r, then the proposition p corresponding to t is assumed to be true. Conversely, if tuple t plausibly could appear in r but doesn’t in fact appear, then the proposition p corresponding to t is assumed to be false. Note: In Chapter 5 I explained The Closed World Assumption in terms of relvars, not relations, but the definition just given is slightly more general. Note that it applies to relations that are the current values of relvars in particular, but it isn’t limited to such relations.

• The Principle of Interchangeability: There must be no arbitrary and unnecessary distinctions between base and virtual relvars.

• The Assignment Principle: After assignment of the value v to the variable V, the comparison V = v must evaluate to TRUE.

• The Golden Rule: No update operation must ever cause the database constraint for any database ...