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CHAPTER 19 THERE S ONLY ONE RELATIONAL MODEL
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Second, Snodgrass s work might indeed be the only temporal database project to have been brought to the attention of the [ANSI] NCITS H2 Committee, but it s certainly not the only such project to have been brought to the attention of the international SQL standards committee. Another such (and one that looks much more promising to me) is reported in among other publications Nikos A. Lorentzos and R. G. Johnson, An Extension of the Relational Model to Support Generic Intervals, in Joachim W. Schmidt, Stefano Ceri, and Michel Missikoff (eds.), Extending Database Technology (Springer Verlag, 1988). Note: Celko s reference to Snodgrass s work is to the book The TSQL2 Temporal Query Language, Richard T. Snodgrass, ed. (Kluwer Academic Publishers, 1995). Lorentzos s work is described in C. J. Date, Hugh Darwen, and Nikos A. Lorentzos: Temporal Data and the Relational Model (Morgan Kaufmann, 2003). Third, Snodgrass s proposals in fact violate the fundamental principle of the relational model viz., The Information Principle and cannot therefore be regarded as being truly relational at all, let alone constituting a different relational model. Note: In case you re unfamiliar with this crucial principle, it runs as follows: All information in the database must be cast explicitly in terms of values in relations and in no other way. Snodgrass s proposals violate this principle because they involve a variety of hidden timestamps on rows in tables.7 Fourth, I d like to comment on the remark that the set models used in all of the data models discussed so far are state oriented and lack a temporal dimension. I don t know what the expression the set models means, nor do I know what it could possibly mean for such models to be used in data models, but I do know that temporal data support has nothing to do with whether the underlying data model is set- or record-level, and I also know that the relational model in particular is perfectly capable of dealing adequately with temporal data (and it doesn t need huge extensions to do it, either). Fifth, the database engine certainly does make a note of when update events occur (admittedly it does so in the log, not in the database, but at least it does do so). But the point is, when update events occur is for the most part not in itself very interesting. What s much more interesting (and much more important) is when certain states of affairs obtain in the real world. That s what we want to record in a temporal database. Sixth, Celko says: The real worry is that the model be consistent with logical constraints on the data. By the model here, I believe he really means the database, not one of his different relational models. And yes, it s vitally important always! that the database be consistent, regardless whether we re dealing with temporal data or any other kind. The significance of his remark in this context escapes me, I m afraid. Seventh: Temporal SQL adds extra clauses to the usual SELECT statement that match rows based on when they occurred relative to each other. What on earth does this sentence mean If Celko really wants to talk about temporal databases, then I think he should make a start by explaining some basic temporal concepts. Then (and only then) he might try to claim that he needs a different relational model. But then, of course, I would dispute that claim (see above).
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How Many Relational Models
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Near the beginning of this chapter, I said the idea of trying to analyze Celko s 18 at a detailed level might be thought a little pointless. But the relational model is so important! and I believe it s important also to try to spread the truth regarding that model as far and wide as
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