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CHAPTER 19 THERE S ONLY ONE RELATIONAL MODEL
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Celko s Preamble
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Let me begin my detailed analysis by repeating Celko s opening sentence (which appears in his preamble, of course): There is no such thing as the relational model for databases anymore [sic] than there is just one geometry. Now, it is of course true that there are several different geometries (euclidean, elliptic, hyperbolic, etc.). But is the analogy that Celko is trying to draw here a valid one That is, do Celko s different relational models differ in the same way that those different geometries differ It seems to me that the answer to this question is no. Elliptic and hyperbolic geometries are often referred to quite specifically as noneuclidean geometries; for the analogy to be valid, therefore, it would seem that at least five of Celko s six different relational models would have to be nonrelational models, and hence (by definition) not relational models at all. Note: Actually, I would agree (and will argue in more detail later) that several of Celko s different relational models are indeed nonrelational. But then he can hardly go on to claim at least, he can t claim consistently that they are then different relational models. So I have a serious problem right away with Celko s overall thesis ... However, let s continue. Here s the next quote: 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. Celko seems to be suggesting here, by analogy, that which of his different relational models is true depends on where he is: a rather curious argument, I would have thought, and one that betrays a strange interpretation of the nature of truth. In any case, I don t think we can reasonably say that a geometry or a model is true ; at best, we might say it s consistent. But we aren t arguing about whether Celko s different relational models are consistent; we re arguing about whether they re really different relational models in the first place.
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Chris Date = No Duplicates, No NULLs
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On to Celko s Section 18.1: Chris [Date s] version [of the relational model] is the simplest and closest to the usual file model of data ... Date s relational model allows no duplicate rows in tables and has no NULL [uppercase NULL in the original]. Well, it s true that my relational model or, better, the relational model! doesn t permit relations to contain duplicate tuples or nulls. That s because a relation that contains duplicate tuples or nulls is not a relation, by definition.1 It follows that a model that permits such relations
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1. Just as an aside, let me say too that a domain that contains a null is not a domain and a tuple that contains a null is not a tuple. See my book An Introduction to Database Systems, 8th edition (Addison-Wesley, 2004), for further explanation.
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CHAPTER 19 THERE S ONLY ONE RELATIONAL MODEL
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is not and cannot be relational (again by definition), and hence that the concept of a relational model that does permit duplicate tuples or nulls is a contradiction in terms. Note: This state of affairs is one of the justifications not the only one for my position that most of Celko s different relational models aren t in fact relational at all. Also, I object strongly to Celko s characterization of my relational model as being close to the usual file model. First of all, I m not even sure what the term the usual file model might mean (though I can guess). Second, and more important, I ve stated as clearly as I can, on many occasions, that we should not think of relations as being somehow like files, but rather as being sets of true propositions. (See, e.g., my book WHAT Not HOW: The Business Rules Approach to Application Development, Addison-Wesley, 2000, for an elaboration of this position.) Indeed, it s precisely because some people do tend to think of relations as being like files that we got into messes like duplicate rows in the first place. To be specific, the idea of a file containing duplicate records seems both familiar and innocuous, whereas (as I ve already indicated) the idea of a relation containing duplicate tuples makes no sense at all. Date is also a vocal opponent of the ANSI/ISO SQL standard efforts, although he has never attended a committee meeting or submitted a paper for consideration. This quote looks dangerously close to being an ad hominem attack, but perhaps I m being oversensitive. Anyway, I ll admit to being a vocal critic (not an opponent ) of the SQL standard (not the SQL standard efforts ). However, I will not admit to using the term the ANSI/ISO SQL standard (I don t think it s appropriate in this context to give one individual national standards body, ANSI, out of many such, a level of billing that is the same as or arguably higher than that of the umbrella organization, which is the international standards body, ISO). As for my participation in the standard efforts, I ll agree that I haven t participated very much, at least not directly (though it s not quite true to say that I ve never attended a committee meeting or submitted a paper for consideration 2). But I do have my reasons for not participating very much. This is not the place to go into those reasons in detail; suffice it to say that they include, but are not limited to, the fact that I couldn t possibly afford the investment required, in terms of either time and money. (I might come back and revisit this particular issue at some future time.) Date has also added other operators and proposed special notation for extensions to an unimplemented language based on the relational algebra. These proposals include the MAYBE postfixed unary logical operator that returns TRUE when its operand is TRUE or UNKNOWN. His SUMMARIZE operator is a generalized columnar function constructor. Well, it s true that, along with my colleague Hugh Darwen,3 I ve proposed a number of extensions I believe useful ones to Codd s original relational algebra. However:
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2. In any case, surely the whole point of a standard is that it is totally defined by the publicly available standard documentation. It is not necessary to attend committee meetings, nor should it be, in order to know what the standard is, or to comment on it, or to criticize it in an informed manner. 3. In passing, let me acknowledge Hugh Darwen s helpful review of an earlier draft of this chapter.
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