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Relational misconceptions don t just abound, they proliferate. They don t seem to die, either. Here are some more examples, taken from letters written after the foregoing article first appeared on the Database Debunkings website. First, some quotes from a message from SW,8 with commentary: A tuple is an ordered list of values, or alternatively an unordered list of key-value pairs.
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8. I ll use this initials-only style to refer to correspondents in Technical Correspondence sections throughout this book.
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CHAPTER 19 THERE S ONLY ONE RELATIONAL MODEL
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No, it s not. A tuple is a set of <A,T,v> triples, where A is an attribute name (and no two triples in the set have the same A component), T is a type name, and v is a value of type T. Immediate consequences: (a) No left-to-right ordering of tuple components. (b) No left-toright ordering of attributes in a relation. (c) No nulls! because nulls aren t values. (d) All relations are in first normal form even if T is (for example) a relation type and v is therefore a relation value. A relation cannot contain duplicates. In practice, I cannot see how this can be the case ... it is impractical to scan every record [sic! a tuple is not a record] whenever a field [sic! an attribute is not a field] changes. Not true. Even today s SQL systems can deal with this requirement (via hashes or indexes), even with very large tables. (This is not to say there aren t better ways to do it.) And even if there is a performance hit in enforcing uniqueness, not enforcing it is worse; it s a suboptimization it makes overall system performance much worse (I m including people performance here, though I don t think it s necessary to do so in order for the argument to be valid). [The] relational model ... does not contain any underlying [ ] representation of relationships ... Alternate approaches ... allow knowledge to be expressed directly ... [and represent] the relationships within the database schema itself. I don t know what SW s background is, but this quote reminds me strongly of Santayana s observation to the effect that those who don t know history are doomed to repeat it. We ve tried DBMSs in the past that represent relationships directly. It was a terrible idea! In fact, I thought it had been completely debunked. But it looks as if all of the old arguments need to be dragged out and dusted off again. Perhaps I ll do that, one of these days. By the way, I hope it s obvious that a relational database does represent relationships; furthermore, it represents them all in the same uniform way. Other databases don t. [An] SQL query is an assertion about a relationship between tables ... It does not seem appropriate for application developers ... to be asserting the semantics of the data model. No, an SQL query is a set definition. The DBMS effectively returns all of the tuples from the database that satisfy the definition. That s all. By the way, SQL is not unique in this regard! Even in IMS, for example, you could ask for (e.g.) the set of employees in a given department. The difference was that you had to do it one tuple at a time, in a loop. Even SQL does it better. [Can] someone provide some pointers [sic!] to any clear comparisons of the pros and cons of the relational model versus the object/relationship [sic] model In Databases, Types, and the Relational Model: The Third Manifesto, 3rd edition (AddisonWesley, 2006), Hugh Darwen and I claim and I think we demonstrate, too that a true object/relational DBMS is nothing more and nothing less than a true relational DBMS. That is, the only sensible interpretation we can give to the term object/relational model is to say that it s just another term for relational model. The trouble is, the term relational has been usurped (destroyed ) by SQL, so the industry needed a new marketing term for systems that implemented a little bit more of the relational model hence the term object/relational DBMS. It s hype, really.
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