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SQL and Relational Theory, 2nd Edition by C.J. Date

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CHAPTER 10

First of all, in the body of the chapter I asked which of the following natural language sentences were legal propositions. My own answers are as follows (but some of them might be open to debate):

  • Bach is the greatest musician who ever lived. Yes.

  • What’s the time? No. It doesn’t make sense to ask “Is it true that p?” where p is “What’s the time?”

  • Supplier S2 is located in some city, x. My own answer here is yes, but you might argue reasonably argue that it should be no. In fact, the example is a good illustration of the ambiguity problem discussed earlier in the chapter; my answer is based on the assumption that the phrase “some city, x” could be abbreviated to just “some city” without changing the overall meaning of the sentence,[208] but you might reasonably argue the opposite. Compare and contrast “We both have the same favorite author, x” (see below).

  • Some countries have a female president. Yes.

  • All politicians are corrupt. Yes.

  • Supplier S1 is located in Paris. Yes (though it’s false, given our usual sample values).

  • We both have the same favorite author, x. No. Here I’m assuming that the phrase “the same favorite author, x” couldn’t be abbreviated to just “the same favorite author” without changing the overall meaning of the sentence (but you could reasonably argue the opposite). If my assumption is correct, then x is a variable (better: a parameter), and until we know what argument is to be substituted for that parameter—i.e., until we know what that x stands for, as it ...

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