Re: RA with MV attributes

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 17 Jan 2007 13:51:02 GMT
Message-ID: <aRprh.1960$1x.32050@ursa-nb00s0.nbnet.nb.ca>

Jon Heggland wrote:

> David wrote:
> 

>>Example
>>
>>r1(Names,Cars)
>> bill, car1,car2,car4
>> john,fred car3
>>
>>r2(Cars,Colours)
>> car1,car3,car4 red
>> car2 green
>>
>>r1 |X| r2 (Names,Cars,Colours)
>> bill car1,car4 red
>> bill car2 green
>> john,fred car3 red
>> john,fred green
>>
>>[...]
>>For example the
>>last tuple of r1 |X| r2 above doesn’t imply that John and Fred
>>don’t own any cars.

> 
> 
> So what exactly does that last tuple mean?

And what would happen if we replaced (car1,car3,car4)<->(red) with (car1,car3,car4)<->(red,blue) in r2 ?

Suppose as well that r2(Cars,Colours) has the following tuple:

                  yellow

Would r1 |X| r2 (Names, Cars, Colours) have these tuples?

bill                               yellow
john, fred                         yellow

Or would it have this tuples?

bill,john,fred                     yellow

What meaning would we ascribe to restricting r2 to the colour yellow and projecting on colour? Similarly, what meaning would we ascribe to restricting the join to the colour yellow and then projecting on Names? Received on Wed Jan 17 2007 - 07:51:02 CST