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There s another way to articulate the foregoing argument, and some readers might like this alternative formulation better. In general, we can regard an SQL SELECT - FROM - WHERE expression as a nested expression, or nested set of function invocations, of the form s ( w ( f ( x ) ) ) Explanation: The expression f(x) f for FROM is a function invocation that returns a table (x here stands for anything that s legal in a FROM clause). w (WHERE) is a function that operates on a table and returns a table. s (SELECT) is a function that operates on a table and returns a table. This hypothetical notation of nested function invocations makes it very clear that, at least conceptually, s can t be invoked before w (because s s input is w s output), and w can t be invoked before f (because w s input is f s output). Thus, if the implementation does want to invoke the functions in some different sequence (perhaps in some kind of coroutine fashion), then it must guarantee that the semantics are preserved; in other words, it must guarantee that it produces the result that would have been produced if the original nested sequence had been adhered to. And again we see, in the example at hand, that this requirement is not met and the implementation is in error.
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3. Actually, another portion of the SQL standard, not quoted here, would require the explicit introduction of a name to refer to the result of the subquery in the FROM clause, like this: SELECT * FROM (...) AS R1 WHERE ... (italics for emphasis) even though that name R1 isn t referenced anywhere else in the query! Oracle, not unreasonably, appears not to enforce this requirement.
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CHAPTER 17 A CURE FOR MADNESS
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Further reflection on Gennick s problem made me realize there was more that could usefully be said on this matter; hence the present section. Let s agree to ignore SQL as such until further notice and switch to relational algebra (and relational terminology) instead. In relational algebra, if r and p are a relational expression and a truth-valued expression, respectively, then the expression r WHERE p evaluates to a relation with the same heading i.e., the same attributes as r and with a body containing just those tuples of r for which p evaluates to TRUE.4 That result relation is said to be a restriction of r, and the operation that computes it is called a restriction operation. Loosely, what the restriction operation does is filter out certain tuples from its operand relation. Now suppose the expression r itself denotes another restriction operation, so that the overall expression takes the form, say, ( s WHERE q ) WHERE p for some relational expression s and some truth-valued expression q. Then it s well known that this overall expression can be legitimately transformed into s WHERE ( q ) AND ( p ) And this transformation is useful for optimization purposes, because the original expression implies that the implementation must perform two passes over the s-relation while the transformed version shows that one pass suffices. Moreover, the original expression suggests that the implementation must check q first when filtering out tuples, but the transformed version shows it can check p first if it likes, because AND is commutative. (And it presumably would like to check p first if, for example, p is easier to evaluate than q, or if it s likely that most tuples satisfy q but very few satisfy p.) Suppose now, however, that we have a situation in which the q-filtering must be done before the p-filtering (as it were). Then the relational expression that represents this state of affairs is not ( s WHERE q ) WHERE p because this expression does not capture the requirement that q must be applied first, as we have just seen. So what is to be done Well, in fact a situation of this same general nature occurs in connection with the issue of type inheritance. Let s look at an example. Note: The discussion that follows is based on one given in the book Databases, Types, and the Relational Model: The Third Manifesto, 3rd edition, by Hugh Darwen and myself (Addison-Wesley 2006), where further details can be found. By way of example, then, let CIRCLE be a subtype of type ELLIPSE, and let relation s have an attribute E that s declared to be of type ELLIPSE. In general, then, some tuples of s will have a value for attribute E that is in fact a circle instead of just a (noncircular) ellipse. Now consider the following query:
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4. Strictly speaking, p must be quantifier-free and its parameters must be some subset of the attributes of r, but there s no need to get into the fine detail of such matters here.
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