MESSAGE
| DATE | 2014-12-11 |
| FROM | Ruben Safir
|
| SUBJECT | Re: Fwd: Re: [LIU Comp Sci] Database Class concepts which are still
|
From owner-learn-outgoing-at-mrbrklyn.com Thu Dec 11 19:04:10 2014
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Date: Thu, 11 Dec 2014 19:04:09 -0500
From: Ruben Safir
To: learn-at-nylxs.com
Subject: Re: Fwd: Re: [LIU Comp Sci] Database Class concepts which are still
unclear
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it can not hurt to be young an chinese an in the first row
On Thu, Dec 11, 2014 at 03:41:25PM -0500, Ruben Safir wrote:
>
>
>
> On 12/11/2014 01:25 PM, Ruben Safir wrote:
> > further along in this chapter:
> >
> > <> > S(X), where
> > the attributes of R are a subset of the attributes of S; >>
> >
>
> I think I got this. It is convulted, but I have it. Everything comes
> down to the one line in the text
>
> T1 ??? ??Y(R)
> T2 ??? ??Y((S ?? T1) ??? R)
> T ??? T1 ??? T2
> SSNS(Ssn) ??? SSN_PNOS ?? SMITH_PNOS
>
> SSN_PNOS R
> Essn Pno
> 123456789 1
> 123456789 2
> 666884444 3
> 453453453 1
> 453453453 2
> 333445555 2
> 333445555 3
> 333445555 10
> 333445555 20
> 999887777 30
> 999887777 10
> 987987987 10
> 987987987 30
> 987654321 30
> 987654321 20
> 888665555 20
>
>
> SMITH_PNOS - which is S
> Pno
> 1
> 2
>
>
> Now
> T1 <-- ?? (ESSN)(SSn_PNOS)
> 123456789
> 666884444
> 453453453
> 333445555
> 999887777
> 987987987
> 888665555
>
>
> That is all possible eligable SSNs
>
> T2 ??? ?? ssn((SMITH_PNOS ?? T1) ??? SSN_PNOS)
>
> Create a cartisian cross of desired pnos with possible sss
>
>
> 123456789 1
> 112456789 2
> 666884444 1
> 666884444 2
> 453453453 1
> 453453453 2
> 333445555 1
> 333445555 2
> 999887777 1
> 999887777 2
> 987987987 1
> 987987987 2
> 888665555 1
> 888665555 2
>
> Now subtract ll the combinations that exist in the real world (R)
>
> 666884444 1
> 666884444 2
> 333445555 1
> 333445555 2
> 999887777 1
> 999887777 2
> 987987987 1
> 987987987 2
> 888665555 1
> 888665555 2
>
>
> and take the projection
>
> 666884444
> 333445554
> 999887777
> 987987987
> 888665555
>
> What is lest is all the values that can not exist as the final set
>
> T ??? T1 ??? T2
> subtract all the possible values from the ones that can not be the value
> and the finished set is the values
> 123456789
> 453453453
>
>
> This is very convoluted. The proof would be interesting to see and
> there are a lot of dependencies and the actualy implementation is very
> difficult, and not worth the time. As a theoretical and practicle
> matter, this is a dead end. You don't do this sort of problem is
> relational math. It do it with Lamda Calculus and for loops.
>
> This is like the quadradic equation. It is a waste of time to teach
> when you can teach them calculus instead and solve the same problem
> without going through 500 steps.
>
> This is what Perl was invented for.
>
> Ruben
>
> > No that is backards S is a subset of R and it says so on the next line
> >
> > <>
> >
> > X is a subset of Z
> > https://en.wikipedia.org/wiki/Subset_and_superset#The_symbols_.E2.8A.82_and_.E2.8A.83
> >
> > On 12/11/2014 12:57 PM, Ruben wrote:
> >> Does anyone understand this. The way this is written it is not
> >> comprehendable. The text doesn't flow logically and example, which in
> >> theory is supposed to explain what the text does not, not not explain it
> >> which leave this and "magic logic" - And Blood of a Nwet and Poof, you
> >> get the desired chicken!
> >>
> >> <<6.3.4 The DIVISION Operation
> >> The DIVISION operation, denoted by ??, is useful for a special kind of
> >> query that
> >> sometimes occurs in database applications.>>
> >>
> >>
> >> Really? What kind?
> >>
> >> << An example is Retrieve the names of employees who work on all the
> >> projects that ???John Smith??? works on. >>
> >>
> >>
> >> What class of query is this called and how is it generlized mathmatically.
> >>
> >>
> >> <>
> >>
> >> OK - we will learn by exmple since we have no definitions
> >>
> >> <> >> in the intermediate relation
> >> SMITH_PNOS:>>
> >>
> >> OK
> >>
> >> <> >> SMITH_PNOS ??? ??Pno(WORKS_ONEssn=SsnSMITH)>>
> >>
> >> OK - that makes sense
> >>
> >> << Next, create a relation that includes a tuple whenever
> >> the employee
> >> whose Ssn is Essn works on the project whose number is Pno in the
> >> intermediate
> >> relation SSN_PNOS:>>
> >>
> >> That is a lot of words that make NO SENSE. This sentence doesn't parse
> >> English Gammar in a way that is meaningful, which is interesting since
> >> this is a chapter on Relational Algrabra for which Grammar is a specific
> >> subset and application of.
> >>
> >> In other words, I don't think the auther knows what he wrote here and
> >> his editor didn't know enough to fix it or clarify it. But we have an
> >> example:
> >>
> >> <>
> >>
> >> Fine this MEANS project all the tuples in WORKS_ON by restricting them
> >> to the attributes of Essn and Pno and assign it to a relationship we
> >> call SSN_PNOS
> >>
> >> <> >> gives the desired
> >> employees??? Social Security numbers:>>
> >>
> >> PRESTO!!! Use this magic symbol and get your desired result!
> >>
> >> <>
> >>
> >> I have no idea what this does and it hasn't been explained. It is a
> >> waste of my time.
> >>
> >> <>
> >>
> >> And this says take a natural join of SSNS and EMPLOYEE and project just
> >> the names.
> >>
> >> Now dissect the example more closely. Maybe "Presto" will show itself
> >> and a logical contruction.
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> SSN_PNOS
> >>
> >> Essn Pno
> >>
> >> 123456789 1
> >>
> >> 123456789 2
> >>
> >> 666884444 3
> >>
> >> 453453453 1
> >>
> >> 453453453 2
> >>
> >> 333445555 2
> >>
> >> 333445555 3
> >>
> >> 333445555 10
> >>
> >> 333445555 20
> >>
> >> 999887777 30
> >>
> >> 999887777 10
> >>
> >> 987987987 10
> >>
> >> 987987987 30
> >>
> >> 987654321 30
> >>
> >> 987654321 20
> >>
> >> 888665555 20
> >>
> >>
> >> SMITH_PNOS
> >> Pno
> >> 1
> >> 2
> >>
> >> SSNS
> >> Ssn
> >> 123456789
> >> 453453453
> >>
> >>
> >>
> >> Magic Results, no explanation. This is the kind of soft material that
> >> litters this class everywhere. I don't understand it and until you can
> >> explain it, I'm not convinced you understand it either.
> >>
> >>
> >> On 12/11/2014 11:03 AM, Ruben Safir wrote:
> >>> 6.2.2 The CARTESIAN PRODUCT (CROSS PRODUCT)
> >>> Operation
> >>> Next, we discuss the CARTESIAN PRODUCT operation???also known as CROSS
> >>> PRODUCT or CROSS JOIN???which is denoted by ??. This is also a binary set
> >>> opera-
> >>> tion, but the relations on which it is applied do not have to be union
> >>> compatible. In
> >>> its binary form, this set operation produces a new element by combining
> >>> every
> >>> member (tuple) from one relation (set) with every member (tuple) from
> >>> the other
> >>> relation (set). In general, the result of R(A1, A2, ..., An) ?? S(B1, B2,
> >>> ..., Bm) is a rela-
> >>> tion Q with degree n + m attributes Q(A1, A2, ..., An, B1, B2, ..., Bm),
> >>> in that order.
> >>> The resulting relation Q has one tuple for each combination of
> >>> tuples???one from R
> >>> and one from S. Hence, if R has nR tuples (denoted as |R| = nR), and S
> >>> has nS tuples,
> >>> then R ?? S will have nR * nS tuples.
> >>> The n-ary CARTESIAN PRODUCT operation is an extension of the above concept,
> >>> which produces new tuples by concatenating all possible combinations of
> >>> tuples
> >>> from n underlying relations.
> >>>
> >>>
> >>> What is an n-ary operation? It is mentioned several times, but I don't
> >>> see an answer
> >>>
> >>> Ruben
> >>>
> >
>
>
>
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