|SUBJECT ||Re: [LIU Comp Sci] Database Class concepts which are still unclear
|From owner-learn-outgoing-at-mrbrklyn.com Thu Dec 11 12:57:43 2014
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Date: Thu, 11 Dec 2014 12:57:44 -0500
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Subject: Re: [LIU Comp Sci] Database Class concepts which are still unclear
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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
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 ? ?Pno(WORKS_ONEssn=SsnSMITH)>>
OK - that makes sense
<< Next, create a relation that includes a tuple whenever
whose Ssn is Essn works on the project whose number is Pno in the
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
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
<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
Now dissect the example more closely. Maybe "Presto" will show itself
and a logical contruction.
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)
> 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
> 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
> 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
> from n underlying relations.
> What is an n-ary operation? It is mentioned several times, but I don't
> see an answer