MESSAGE
| DATE | 2014-12-11 |
| FROM | Ruben Safir
|
| SUBJECT | Re: [LIU Comp Sci] Database Class concepts which are still unclear
|
From owner-learn-outgoing-at-mrbrklyn.com Thu Dec 11 13:25:09 2014
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Date: Thu, 11 Dec 2014 13:25:12 -0500
From: Ruben Safir
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To: learn-at-nylxs.com, Ping-Tsai Chung
Subject: Re: [LIU Comp Sci] Database Class concepts which are still unclear
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further along in this chapter:
<S(X), where
the attributes of R are a subset of the attributes of S; >>
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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