MESSAGE
| DATE | 2014-12-12 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [LIU Comp Sci] Re: turning attibute values into relations?
|
From owner-learn-outgoing-at-mrbrklyn.com Fri Dec 12 17:35:58 2014
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Date: Fri, 12 Dec 2014 17:36:04 -0500
From: Ruben Safir
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To: learn-at-nylxs.com, Ping-Tsai Chung
Subject: [LIU Comp Sci] Re: turning attibute values into relations?
References: <5489C901.7040301-at-mrbrklyn.com>
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On 12/12/2014 06:50 AM, Lennart Jonsson wrote:
> On 2014-12-11 17:40, ruben safir wrote:
>> I know that usenet is bad for homework but this problem is driving me
>> crazy
>>
>> I need some expertise outside my current ring of contacts.
>>
>> This question is
>> (e) Retrieve the names of employees who work on every project
>>
>> Which I think is saying find the employees that work on ALL projects.
>>
>
> What you are looking for is relational division. Using the universal
> quantifier ? (not sure if this i shown correkt so I'll use Forall below)
>
> Forall x:p(x) where x is project and p is "employee works on". This is
> not directly supported in SQL, but can be rewritten using Existential
> quantification:
>
> Forall x:p(x) <=> NOT Exists x : NOT p(x)
>
> I.e. employees for which there is no project such that the employee is
> not working on it. This is relatively simple to transform to SQL.
>
> For SQL you could also use an approach with employees that work on the
> same number of projects as there are projects
>
>
>
> /Lennart
>
>
>
>
>> How do you turn a relationship into an atribute that can be quried on
>>
>> This is the relavent relations (tables)
>> SQL> descr employee
>> Name Null? Type
>> ----------------------------------------- --------
>> ----------------------------
>> FNAME NOT NULL VARCHAR2(20)
>> MINIT CHAR(1)
>> LNAME NOT NULL VARCHAR2(20)
>> SSN NOT NULL NUMBER(9)
>> BDATE NOT NULL DATE
>> ADDRESS NOT NULL VARCHAR2(50)
>> SEX NOT NULL CHAR(1)
>> SALARY NOT NULL FLOAT(12)
>> SUPERSSN NUMBER(9)
>> DNO
>> NUMBER(2)
>>
>>
>> SQL> descr PROJECT
>> Name Null? Type
>> ----------------------------------------- --------
>> ----------------------------
>> PNAME NOT NULL VARCHAR2(20)
>> PNUMBER NOT NULL NUMBER(2)
>> PLOCATION NOT NULL VARCHAR2(15)
>> DNUM
>> NUMBER(2)
>> SQL> descr works_on
>> Name Null? Type
>> ----------------------------------------- --------
>> ----------------------------
>> ESSN NOT NULL NUMBER(9)
>> PNO NOT NULL NUMBER(2)
>> HOURS FLOAT(5)
>>
>>
>>
>> So you want a query that includes off the essn's in workson that
>> associated with all the possible pno and match pnumbers in projects.
>>
>> I tried this in SQL and in relational algebra and I can't see a path to
>> a solution.
>>
>> Ruben
>>
>
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