Re: teaching relational basics to people, questions

From: <compdb_at_hotmail.com>
Date: Fri, 4 Dec 2009 16:26:42 -0800 (PST)
Message-ID: <35bb0b6c-af79-4c41-beb1-34fe00e81643_at_y32g2000prd.googlegroups.com>


On Nov 29, 12:31 am, "Mr. Scott" <do_not_re..._at_noone.com> wrote:
> I am going to revisit the example I posted for paul.

Thank you for giving an example. Especially when my stated intent is to show you misunderstand something.

Your explanation is still unclear.

The user looks at the world and the relation variable's predicate(s) and combines them to determine what rows to put in and what rows to leave out. Similarly, they look at the rows present and absent in a relation variable and combine them with the predicate(s) to produce statements about the world. Please tell me what the designer gives with a relation variable (a predicate, or set of predicates?) and the correspondence between an overall proposition about the world and the rows in and the rows not in that variable (now and/or previously).

> CTRS {COURSE, TEACHER, ROOM, STUDENT},

> each row states that a particular COURSE is taught by a particular TEACHER
> in a particular ROOM to a particular STUDENT.

> one effectively asserts
> that 'there is a course <COURSE>,'
> that 'there is a teacher <TEACHER>,'
> [...]

The designer gives a relation variable. It's not clear what else you claim they give. A single predicate per variable? A set of predicates per variable? Do you mean that (some function of) this list is a clarification of the preceding (parameterized) statement? If so, exactly what predicate does the user form from the list that is supposed to be equivalent to the statement? Or do you mean that the designer specifies the statement and you think the list is derivable from the statement by the system?

I can't tell when you write "when one inserts a row" or "inserting a single row", whether you just mean mean "when a given row is in a relation". Or whether by "inserting a single row" or "deletes that only row" you specifically mean the old value of a relation variable is also involved in mapping to instantiated predicates.

> Either all of the atomic formulas
> represented by the row are true, or none of them are. That is consistent
> with the logical connective between those formulas being IFF rather than
> AND.
First you write "one effectively asserts" the things in the list (whatever "effectively" means); asserting a number of things is the same as asserting their conjunction. But then you write "the logical connective between those formulas being IFF", so I guess you mean the IFF of the things in the list is asserted. So you're not being clear.

And no, it's not consistent with the logical connective between those formulas being IFF. For example, although   a1 IFF a2 IFF a3
is true when all ai are true and it is false when all ai are false, it is also true when a1 and a2 are false and a3 is true. So the above expression does not assert that they're all true or all false. For that you would want something like
  (a1 AND a2 AND a3) OR ((NOT A1) AND (NOT a2) AND (NOT a3)) ie (NOT (a1 OR a2 OR a3)) OR (a1 AND a2 AND a3) ie (a1 OR a2 OR a3) IMPLIES (a1 AND a2 AND a3) ie the whole thing is true when all are false or all are true but it's false when there's a mix. (Not that I think you ever want the ai all individually false.)

On the other hand I can imagine that you are thinking that for each syntactically valid row it is in the relation iff all the corresponding instantiated assertions in the list are true. But that's not what you have said. (In fact you've said "all of the atomic formulas represented by the row are true, or none of them are".) And anyway that's not a relation's statement about the world; it is a statement about how the value of a relation variable maps to its statement about the world.

> If
> The information content of a table
> is the logical sum (disjunction) of the information represented by each row.

This is clearer, but as I said I can't figure out what you think is asserted by a row in the table (or by a row not in 'the table) to be ORed together. (Note that it makes sense to OR together equalities like COURSE='Math' as the characteristic predicate of a set, but to AND together things like "there is a course 'Math'" as a statement or assertion about the world.) But I also don't know whether you mean this applies to all relation variable values, or only the values of ones that that are empty, or just became empty or nonempty, or didn't, or what.

So please start out with what the designer gives, then tell me clearly how to form the proposition is that is "the information represented by each row" that is syntactically valid, then what the proposition is that is "the information content of a table". And whether it depends on the old state as well as the new. And tell me whether I am using that predicate as a statement about the world or as the characteristic predicate of some set. In other words, be clear. Then I can discuss your approach (ie show it doesn't do what you think it does).

It's also pointless for me to address your justifications for individual steps if I don't understand the overall process.

philip Received on Sat Dec 05 2009 - 01:26:42 CET

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