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CHAPTER 19 THERE S ONLY ONE RELATIONAL MODEL
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a. Celko s choice of MAYBE as an example of such an extension is bizarre in the extreme! First of all, MAYBE isn t an extension to the algebra, as such, at all; as Celko himself says, it s a logical operator, not a relational one. Second, his definition is wrong (MAYBE returns TRUE when its operand is UNKNOWN, not TRUE or FALSE). Third, MAYBE is a prefix operator, not a postfixed one. (Just how carefully did Celko read what I had to say on this particular topic, I wonder ) Fourth and much the most important point I didn t propose the MAYBE operator at all! Rather, I simply pointed out that if you want to support nulls (which I categorically don t want to do), then you have to support the MAYBE operator (and then I went on to explore some of the implications of that state of affairs). That s all. b. I don t know what a generalized columnar function constructor might be, but I m pretty sure SUMMARIZE isn t one. Au contraire, it s a clean, orthogonal, non-ad-hoc analog of SQL s grouping-with-aggregation operation. In any case, SUMMARIZE was originally due, not to me, but as far as I know to the people who developed the PRTV prototype at IBM U.K. in the early 1970s. See Stephen Todd s paper The Peterlee Relational Test Vehicle A System Overview, IBM System Journal 15, No. 4 (1976); see also the paper by Patrick Hall, Peter Hitchcock, and Stephen Todd An Algebra of Relations for Machine Computation, which appeared in the Conference Record of the 2nd ACM Symposium on Principles of Programming Languages, Palo Alto, Calif. (January 1975). c. Proposed special notation for extensions to an unimplemented language, eh What exactly is the reader supposed to make of this wording I think it would be fairer (not to mention being more accurate and less of an apparent putdown) to say rather that Hugh Darwen and I have defined additional relational operators and then, in order to explain those operators, we have invented some syntax to use as the basis for examples to illustrate their use. After all, when Codd defined his original relational algebra, he too proposed special notation for an unimplemented language. (I might add that Codd s algebra remains essentially unimplemented to this day, at least in commercial form. I might add further that this state of affairs is a major reason why today s commercial products are so plainly unsatisfactory.) d. Just to add insult to injury, Celko then goes on to give a reference that he says explains MAYBE and SUMMARIZE and the reference is incorrect! Just for the record, you can find such explanations in my book An Introduction to Database Systems, 8th edition (AddisonWesley, 2004). That book in turn includes references to the original source documents.
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E. F. Codd, RM Version I
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Dr. Codd s original model of the relational database has been given the name RM Version I ... This classic model allowed no duplicate rows and [had] one NULL ... Actually it was Codd himself who gave his original model the name RM Version 1 (that s supposed to be an Arabic 1 , by the way, not a Roman I ). The expression model of the relational database is a curious one what can it mean but the somewhat circular model for databases that are constructed according to the relational model The expression classic model is odd too; indeed, I don t really know what it means.
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CHAPTER 19 THERE S ONLY ONE RELATIONAL MODEL
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Anyway, Codd s RM/V1 did indeed allow no duplicate rows. It also had no nulls! In his book The Relational Model for Database Management: Version 2 (Addison-Wesley, 1990), Codd says: I refer to the total content of [my] pre-1979 papers as Version 1 of the relational model (RM/V1 for brevity). The extensions to RM/V1 to support nulls weren t defined until the 1979 paper Extending the Database Relational Model to Capture More Meaning, ACM TODS 4, No. 4 (December 1979). Note, therefore, that the relational model managed perfectly well without nulls for over ten years (Codd s very first paper on the relational model, Derivability, Redundancy, and Consistency of Relations Stored in Large Data Banks, IBM Research Report RJ599, was dated August 19th, 1969).
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