MESSAGE
| DATE | 2014-12-11 |
| FROM | Ruben Safir
|
| SUBJECT | Fwd: Re: [LIU Comp Sci] Database Class concepts which are still unclear
|
From owner-learn-outgoing-at-mrbrklyn.com Thu Dec 11 15:41:21 2014
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Date: Thu, 11 Dec 2014 15:41:25 -0500
From: Ruben Safir
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Subject: Fwd: Re: [LIU Comp Sci] Database Class concepts which are still unclear
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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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