1. Database Truth

"The RDM is a formal system. It has two parts. Semantics its outside the formal language (which is Deductive Subsystem), but not outside the interpretation (i.e., application) of that language (Interpretation Subsystem). Without an Interpretation Subsystem there is no possibility of applying the formal system and it remains an abstract game of symbols."

"Codd's 1979 paper described a way to "capture" semantics using the relational formalism. That formalism doesn't tell you how to discover semantics, but if you have them, then he showed (at least to some degree) how to express those semantics relationally."

"Semantics is about applying the RDM to some subject. In effect, what you do is restrict the power of the abstract formalism so that it is more closely aligned with your intended use. In my terminology, that means you:

• Create axioms (expressed as constraints), limiting the vocabulary to the subject matter (and making it finite and usually fairly small); and,
• Restrict the possible interpretations that can be used consistently with the resulting subset of the formalism."

2. Do You Know What's Wrong With This Picture?

"When someone refers to a relation in a database course, what does that mean?"

"It means that it is time to go to Wikipedia."

"A relation in the context of modeling a problem will include the fields and possibly the identification of fields which have relationships with other relations."

"A relation is a data structure which consists of a heading and an unordered set of tuples which share the same type."

"I was trying to show that, in SQL, a relation is more than just a table. Queries return relations. And within a query, relational math is happening, with many intermediate results, that themselves are relations."

"A relation is an abstract structure which contains a set of attributes, and a relvar is the dataset status in a particular moment of this relation. The first one can be considered as the table definition with columns, and the second one is dataset in this table." 

"Tuples need not have a key (or any way of locating them?) Having tried to answer this question so that I could explain it to my students, I am forced to the conclusion that the theory has nothing whatsoever to do with "data" in the usual sense. Perhaps information Theory would have been a better basis for data systems, rather than Mathematics? Computer applications are notably different from Physics, which math was created to model, and its child, Engineering. I think data was never intended to be "true", it must be useful."

Abstract

• The relational key concept
• The kinds of relational keys, their properties and functions
• The formal PK mandate
• PK selection
• RDBMS key support