Re: Multiple keys and transition constraints
From: J M Davitt <jdavitt_at_aeneas.net>
Date: Sat, 16 Sep 2006 03:54:55 GMT
Message-ID: <jAKOg.222$8e5.105_at_tornado.ohiordc.rr.com>
>>"JOG" <jog_at_cs.nott.ac.uk> wrote in message
>>news:1158251660.983027.98520_at_e3g2000cwe.googlegroups.com...
>>
>>>Brian Selzer wrote:
>>>
>>>>Given a relation schema R {A, B, C}, where A and B are each candidate
>>>>keys.
>>>>
>>>>If the current extension is
>>>>
>>>>r {{A:1, B:9, C:3}
>>>> {A:2, B:8, C:4}}
>>>>
>>>>and the proposed extension is
>>>>
>>>>r' {{A:1, B:8, C:4}
>>>> {A:2, B:9, C:3}}
>>>>
>>>>is only A different? Or are both B and C different? From one
>>>>perspective,
>>>>both B and C remain constant but A is different. From another
>>>>perspective,
>>>>A remains constant but both B and C are different. If you're trying to
>>>>specify the allowable transitions for C, which key do you use? A, B, the
>>>>superkey, AB, or none of the above?
>>>
>>>This requires more information to answer.
>>>
>>>Can you tell us which attribute identifies the subject [of each
>>>proposition] in real life over the updates. Then I can determine /what/
>>>exactly has experienced the transition.
>>>
>>>No other questions, just that.
>>>
>>
>>Well, that's the dilemma, there are two subjects because there are two keys.
>>Consider the following relation:
>>
>>EMPPOS {EMP#, RATE, POS#}
>>
>>where EMP# refers to an employee (presumably specified in an EMPLOYEE
>>relation which has information like name, address, etc.) and POS# refers to
>>a position in the company (presumably specified in a POSITION relation which
>>has information like job description, shift, etc.). RATE has two different
>>meanings that coincide: from one perspective it's an employee's current pay
>>rate; from the other it's what's currently being paid for a position in the
>>company. Now assume that employee #1 was switched to position #8 which is
>>on the evening shift and employee #2 was switched to position # 9 which is
>>on the day shift. So, from the perspective of positions, only EMP# is
>>different--employee #1 is now filling position #8 and employee #2 is now
>>filling position #9. From the perspective of employees, both RATE and POS#
>>are different--employee #1 got a $1/hr pay increase and employee #2 got a
>>$1/hr pay cut.
Date: Sat, 16 Sep 2006 03:54:55 GMT
Message-ID: <jAKOg.222$8e5.105_at_tornado.ohiordc.rr.com>
JOG wrote:
> Brian Selzer wrote: >
>>"JOG" <jog_at_cs.nott.ac.uk> wrote in message
>>news:1158251660.983027.98520_at_e3g2000cwe.googlegroups.com...
>>
>>>Brian Selzer wrote:
>>>
>>>>Given a relation schema R {A, B, C}, where A and B are each candidate
>>>>keys.
>>>>
>>>>If the current extension is
>>>>
>>>>r {{A:1, B:9, C:3}
>>>> {A:2, B:8, C:4}}
>>>>
>>>>and the proposed extension is
>>>>
>>>>r' {{A:1, B:8, C:4}
>>>> {A:2, B:9, C:3}}
>>>>
>>>>is only A different? Or are both B and C different? From one
>>>>perspective,
>>>>both B and C remain constant but A is different. From another
>>>>perspective,
>>>>A remains constant but both B and C are different. If you're trying to
>>>>specify the allowable transitions for C, which key do you use? A, B, the
>>>>superkey, AB, or none of the above?
>>>
>>>This requires more information to answer.
>>>
>>>Can you tell us which attribute identifies the subject [of each
>>>proposition] in real life over the updates. Then I can determine /what/
>>>exactly has experienced the transition.
>>>
>>>No other questions, just that.
>>>
>>
>>Well, that's the dilemma, there are two subjects because there are two keys.
>>Consider the following relation:
>>
>>EMPPOS {EMP#, RATE, POS#}
>>
>>where EMP# refers to an employee (presumably specified in an EMPLOYEE
>>relation which has information like name, address, etc.) and POS# refers to
>>a position in the company (presumably specified in a POSITION relation which
>>has information like job description, shift, etc.). RATE has two different
>>meanings that coincide: from one perspective it's an employee's current pay
>>rate; from the other it's what's currently being paid for a position in the
>>company. Now assume that employee #1 was switched to position #8 which is
>>on the evening shift and employee #2 was switched to position # 9 which is
>>on the day shift. So, from the perspective of positions, only EMP# is
>>different--employee #1 is now filling position #8 and employee #2 is now
>>filling position #9. From the perspective of employees, both RATE and POS#
>>are different--employee #1 got a $1/hr pay increase and employee #2 got a
>>$1/hr pay cut.
> > > Okay so initially: > > r = { {emp:1, position:9, pay:3} > {emp:2, position:8, pay:4} } > > and then later: > > r' = { {emp:1, position:8, pay:4} > {emp:2, position:9, pay:3} } > > What problem is there with this? All looks fine to me.
That's right!
Me, too!
What he said!
There you go!
Uh-huh!
Yup!
Yes!
Enough?
Received on Sat Sep 16 2006 - 05:54:55 CEST