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CHAPTER 6 ON THE LOGICAL DIFFERENCES BETWEEN TYPES, VALUES, AND VARIABLES
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Given a relation R, we say that attribute Y of R is functionally dependent on attribute X of R if and only if each X value in R has associated with it precisely one Y value in R (at any one time). I should have said relvar, not relation! then I could (and should, and would) have deleted (at any one time). In other words: If we call a relvar a relation, what do we call a relation Confusions have also arisen over update. In the sixth edition of that same book (though only in the first printing!), I wrote: Tuple assignments are performed (implicitly) during [relational] UPDATE operations. No, they re not! As we know, relational UPDATE operations and INSERT and DELETE operations too, of course are really shorthands for certain relational assignments, which replace the current value (a relation) of the target relvar by another value (another relation). The target of such assignments is categorically not a tuplevar (as it would have to be in a genuine tuple assignment) not even in the special case where we re trying to DELETE or UPDATE a relvar that currently contains just one tuple. Further examples of confusion over updating are provided by SQL. Consider first the SQL INSERT statement. In its simplest and probably commonest form, that statement looks like this: INSERT INTO <table name> ( <column name commalist> ) VALUES ( <expression commalist> ) ; For example: INSERT INTO EMP ( EMP#, DEPT#, SALARY ) VALUES ( EMP#('E1'), DEPT#('D1'), MONEY(50000) ) ; This example would widely be regarded as inserting a row into table EMP, but it really shouldn t be! see the discussion of INSERT in the section Relation Values and Variables earlier in this chapter. A more serious issue is raised by the positioned forms of the DELETE and UPDATE statements (i.e., DELETE and UPDATE via a cursor). For example: UPDATE EMP SET SALARY = SALARY * 1.1 WHERE CURRENT OF EXC ; This statement is defined to be a single-row operation it updates the particular row that cursor EXC happens to be positioned on (I m assuming that EXC here is a cursor running over the EMP table, of course). But updating a row means by definition that the row in question must be a row variable! and there s no such thing, in the relational model. In other words, I m suggesting that SQL s positioned DELETE and UPDATE statements are and always were a logical mistake. In fact, they would probably never have been included in the language in the first place if the original SQL designers had understood the logical difference between relation (or table) values and variables. By the way, it s relevant to mention in passing that certain cursor-based updates can never work, anyway. As a trivial example, suppose table EMP is subject to the constraint that employees E1 and E2 must always be in the same department. Then any cursor-based (and hence single-row) update that attempts to change the department number for either E1 or E2 will necessarily fail.
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CHAPTER 6 ON THE LOGICAL DIFFERENCES BETWEEN TYPES, VALUES, AND VARIABLES
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Let me now turn to the issue of confusion over types and either values or variables or both. I ve already alluded to the fact that a failure to distinguish between types and variables in fact, between scalar types and relation variables, if you think about it! constitutes The First Great Blunder. I don t want to discuss that blunder any further here, except to stress the point that it really is a blunder of huge proportions ... If you want to learn more about it, a detailed discussion can be found in An Introduction to Database Systems. But I do want to say something about The Second Great Blunder, which can be characterized, informally, as mixing pointers and relations that is, allowing an attribute in one relation to contain pointers to tuples in another. Now, I hope you realize immediately that this notion makes no sense, because pointers to tuples has to mean pointers to tuple variables, and there s no such thing as a tuple variable in a relational database (hence the quotation marks in the previous paragraph). So we really are talking about another huge blunder ... Again, I don t want to discuss the issue in detail here (if you re interested, you can find such a discussion in An Introduction to Database Systems). Instead, I just want to make the following points: A system that commits the first great blunder is inevitably led into committing the second as well. That is, the second is a logical consequence of the first, and so it s also, logically, a consequence of confusing types and variables. However, a system that avoids the first great blunder can still commit the second, anyway. To that extent, therefore, the second is independent of the first. Since we ve already seen that the second involves pointers to tuples, however, we can say that the second is also a consequence of confusing values and variables, as well as a consequence of confusing types and variables. Third, as I ve written elsewhere, if SQL doesn t quite commit the two great blunders, it certainly sails very close to the wind ... To be specific, SQL does include support for REF types, it does include support for dereferencing (though, oddly, not for referencing), and it does allow rows in one table to contain references to rows in another. Now, advocates of these SQL features claim that they (i.e., the features in question) are all really just shorthand for various combinations of features that already exist in the language. But even if we accept this claim as valid (which it patently isn t, by the way), the fact remains that the new features have the effect of making SQL much more complicated than it was before as if it wasn t complicated enough already! and (to say it again) they re all consequences, in the last analysis, of confusing types, values, and variables.
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