MESSAGE
| DATE | 2014-12-18 |
| FROM | Ruben Safir
|
| SUBJECT | Re: [LIU Comp Sci] Normalization review
|
From owner-learn-outgoing-at-mrbrklyn.com Thu Dec 18 10:54:50 2014
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Date: Thu, 18 Dec 2014 10:55:04 -0500
From: Ruben Safir
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To: learn-at-mrbrklyn.com
Subject: Re: [LIU Comp Sci] Normalization review
References: <20141218153424.GA4242-at-panix.com> <20141218154519.GC4242-at-panix.com>
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On 12/18/2014 10:50 AM, Sergei Glikstein wrote:
> where is that?
On the B train Kings Highway Stop...
There is coffee and Piazza here.
1580 East 19th Suite 1E
first floor
Can anyone explain this?
6.6.7 Using the Universal Quantifier in Queries
Whenever we use a universal quantifier, it is quite judicious to follow
a few rules to
ensure that our expression makes sense. We discuss these rules with
respect to the
query Q3.
Query 3. List the names of employees who work on all the projects controlled
by department number 5. One way to specify this query is to use the
universal
quantifier as shown:
Q3: {e.Lname, e.Fname | EMPLOYEE(e) AND ((?x)(NOT(PROJECT(x)) OR NOT
(x.Dnum=5) OR ((?w)(WORKS_ON(w) AND w.Essn=e.Ssn AND
x.Pnumber=w.Pno))))}
6.6 The Tuple Relational Calculus
181
We can break up Q3 into its basic components as follows:
Q3: {e.Lname, e.Fname | EMPLOYEE(e) AND F }
F = ((?x)(NOT(PROJECT(x)) OR F1))
F1 = NOT(x. Dnum=5) OR F2
F2 = ((?w)(WORKS_ON(w) AND w.Essn=e.Ssn
AND x.Pnumber=w.Pno))
We want to make sure that a selected employee e works on all the
projects controlled
by department 5, but the definition of universal quantifier says that to
make the
quantified formula TRUE, the inner formula must be TRUE for all tuples
in the uni-
verse. The trick is to exclude from the universal quantification all
tuples that we are
not interested in by making the condition TRUE for all such tuples. This
is necessary
because a universally quantified tuple variable, such as x in Q3, must
evaluate to
TRUE for every possible tuple assigned to it to make the quantified
formula TRUE.
>
> On Thu, Dec 18, 2014 at 10:45 AM, Ruben Safir wrote:
>> On Thu, Dec 18, 2014 at 10:39:29AM -0500, Sergei Glikstein wrote:
>>> i am in the library upstairs
>>>
>>> On Thu, Dec 18, 2014 at 10:34 AM, Ruben Safir
>> wrote:
>>>> Anyone want to review normalization?
>>>>
>>>> Ruben
>>>>
>>>>
>>
>> why not come here so I can pick your brain without distractions and with
>> a network that functions.
>>
>> Everytime i go to that damn libvrary i waste my time
>>
>> Ruben
>>
>> BTW - you know IRC - xchat?
>>
>> Ruben
>>
>>
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