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What are the consequences of confusing values and variables Well, here s one. Consider the following quote (it s taken from an article entitled On Marrying Relations and Objects: RelationCentric and Object-Centric Perspectives, by Won Kim, in Data Base Newsletter 22, No. 6, November/December 1994): [Some] relational database systems ... assign tuple identifiers to tuples in relations ...
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CHAPTER 6 ON THE LOGICAL DIFFERENCES BETWEEN TYPES, VALUES, AND VARIABLES
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I note in passing that relational here should probably be SQL and relations should certainly be relvars, but I don t want to pursue those issues further here. Rather, the point is that the systems in question are clearly thinking of the relvars in question as consisting of collections of tuple variables or tuplevars there wouldn t be any point in assigning identifiers to (or, better, associating identifiers with) tuple values! So the systems in question are based on a logically flawed concept; as we know, relvars don t contain tuplevars, they contain relation values, which contain tuple values. Even this flaw might not matter, however (I mean, it might not be a concern at the model level), were it not for the following: First, the systems in question typically expose those tuple IDs to the user (indeed, if they don t, then from the user s point of view those tuple IDs don t exist they aren t part of the model, and we don t need to talk about them). Exposing them to the user means the user can access and exploit them in a variety of ways. Unless they re represented to the user as just another relational attribute, however (which they might or might not be), we have a violation of The Information Principle on our hands. Second, so far as I m aware, the systems in question universally associate such IDs with tuples in base relvars only, not with tuples in other kinds of relvar (e.g., views), and certainly not with tuples in relations produced as the result of evaluating some general relational expression (e.g., the result of some query). Users thus see a logical difference between base relvars and other kinds, and between base relations and other kinds (where by the term base relation I mean a relation that happens to be the value of some base relvar). And so we have a violation of The Principle of Interchangeability on our hands. Note: The Principle of Interchangeability (of base and derived relvars) is discussed in depth in my book An Introduction to Database Systems. Basically what it says is simply that the user shouldn t be able to tell the difference between base relvars and other kinds. Following on from the foregoing discussion, I now claim that it was at least partly a confusion over the value vs. variable distinction that led to the introduction of object IDs into the object world. The Third Manifesto rejects the concept of object IDs, finding them to be both unnecessary and undesirable. The argument to show they re unnecessary goes like this: By definition, every value is distinct from every other value (i.e., is in fact self-identifying). Values thus have no need to carry around with themselves some hidden, secret identifier that s somehow separate from the value itself, and indeed they don t do so. Variables, by contrast, do need some identity that s separate from their current value, and that identification is provided, precisely and sufficiently, by the variable s name. As for the undesirability of object IDs, it s sufficient here to say simply that they re logically indistinguishable from pointers, and I ve already said we don t want pointers in the database. (I don t think I need to give a detailed justification of this latter position here, though I might do so in some future paper.) Of course, the relational world too has been guilty in the past of some confusion over values and variables: over relation values and variables, to be specific. And that fact has led to some confusion over normalization in particular. In the second edition of my book An Introduction to Database Systems, for example, I wrote:
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