Re: By The Dawn's Normal Light

From: Bill H <wphaskett_at_THISISMUNGEDatt.net>
Date: Tue, 26 Oct 2004 19:38:17 GMT
Message-ID: <JIxfd.319330$D%.238237_at_attbi_s51>


"Laconic2" <laconic2_at_comcast.net> wrote in message news:v7KdnTYJA9OV2uTcRVn-hg_at_comcast.com...

[snipped]

> The third is that the values might be sets.
>
> SELECT s.id, s.name
> FROM enrollments s
> WHERE s.courses_enrolled = {'PSYCH101', 'ENG101', 'MATH101', 'CS101'}
>
> Now the question of whether courses_enrolled is a "set" meaning unordered,
> or a "list" meaning ordered is crucial for the correct interpretation of
> the equality test.
>
> Now the question of whether courses_enrolled is a "set" meaning unordered,
> or a "list" meaning ordered is crucial for the correct interpretation of
> the equality test.
>
> If a student is enrolled in these courses: {'CS101', 'PSYCH101', 'ENG101',
> 'MATH101'} the student has the same SET of courses as the criterion. But
> does not have the same LIST of courses as the criterion. Thus the
> distinction between a SET and a LIST is critical to the type engine. IMO,
> of course.
>
> If I got it right, it is this third case that distinguishes the RDM from
> the Nelson-Pick model.

You are very correct in identifying the Nelson-Pick db model as one where a LIST may be unconcerned with the order.

When we define the "type engine" as a data manipulation process outside the dbms the Nelson-Pick model more closely matches your thinking than you might imagine. The Nelson-Pick query is _very_ likely to return a list in different order than the query and the equality operator can certainly recognize this distinction. Do I, therefore, define a SET and LIST as different. This may or may not be germane depending on our question:

  1. Are we taking the same classes? Yes.
  2. Are we in the same classes? Maybe.

I suspect Dawn would point out that the definitions don't restrict the reality. I would point out that some models are easier to think about than others and work with than others and these comparisons change depending on what we're doing.

An obvious non-rdbms question would be: If a student is enrolled in these courses: {'PSYCH101', 'ENG101', 'MATH101', 'CS101'} the student has the same SET of courses as the criterion. Does the student have the same LIST of courses as the criterion. :-)

Bill Received on Tue Oct 26 2004 - 21:38:17 CEST

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