Re: Question re: Practical Issues in Database Management
From: Joe Thurbon <usenet_at_thurbon.com>
Date: Sun, 25 Mar 2007 21:48:52 GMT
Message-ID: <87CNh.1665$M.1590_at_news-server.bigpond.net.au>
>
> The representation
>
>
> doesn't capture that subtlety. It suggests ACTIVITY
> as atomic, not SET_OF_ACTIVITIES.
>
>
> Either you are making the same mistake as I or we are both right :-)
>
>
> http://www.almaden.ibm.com/cs/people/fagin/tods77.pdf
>
> Maybe, maybe not. Again, your notation suggests no.
> Reconsider what your notation does and does not convey.
>
Date: Sun, 25 Mar 2007 21:48:52 GMT
Message-ID: <87CNh.1665$M.1590_at_news-server.bigpond.net.au>
mAsterdam wrote:
> Joe Thurbon wrote:
>> TroyK wrote: >>> Joe Thurbon wrote: >> >> [...] >> >>>> "A MVD between two columns exists when sets of values in one column are >>>> each associated with values in another column" >>>> >>>> and 4th normal form is defined as >>>> >>>> "If no MVDs exist between columns, then a table is in 4th normal form" >>>> >>>> Even assuming that 'no MVDs' is shorthand for 'no MVDs that are not >>>> also >>>> FDs' these definition would mean that the table, >>>> >>>> EMP# ACTIVITY >>>> ============== >>>> 130 DEBUG >>>> 130 SUPPORT >>>> >>>> would not be in 4th normal form. (Since, it's clear that there is a set >>>> of values {DEBUG, SUPPORT} that is functionally dependant on EMP#. The >>>> caption of the example on page 138 says that the above table is in 4th >>>> normal form (as do all other definitions I've read). >>> >>> But the set of values is not, in fact, functionally determined by emp >>> #. >> >> I disagree (he says, with trepidation). >> >> Although I may be abusing the term functionally dependent - I'd >> welcome correction if I am. Note above that I said there is a _set_ of >> values, and I meant that that set is functionally dependant on EMP#, >> not that ACTIVITY was functionally dependent on EMP#.
>
> The representation
>
>> EMP# ACTIVITY >> ============== >> 130 DEBUG >> 130 SUPPORT
>
> doesn't capture that subtlety. It suggests ACTIVITY
> as atomic, not SET_OF_ACTIVITIES.
>
>> I thought that the general notion of dependency was (informally) along >> the lines of: >> >> "B is dependent on A if, every time I am told a value for A, I can >> observe the world and determine the value of B."
>
> Either you are making the same mistake as I or we are both right :-)
>
>> For functional dependencies, (A -> B), B is a 'simple value'. For >> multivalues dependencies, (A ->> B), B is a set of values, and for >> join dependencies, I'll have to keep reading and see if my analogy >> falls over.
>
> http://www.almaden.ibm.com/cs/people/fagin/tods77.pdf
Thanks for the pointer.
>
>> As an aside, for a table in 3NF, for exmaple. >> >> TABLE A B >> === --- >> 1 1 >> 2 1 >> 3 2 >> >> Is it right to say that >> >> A -> B >> >> and >> >> B ->> A >> >> And if not, why not?
>
> Maybe, maybe not. Again, your notation suggests no.
> Reconsider what your notation does and does not convey.
>
Just to be clear, the notation isn't mine. I just repeated it from "Practical Issues" (e.g. page 196). I think that B ->> A is intended only convey that for each B, there are several A's associated with it.
> Googling for "population diagram" (by far the most
> important data analysis tool according to Nijssen)
> did not give me what I was looking for (a good tutorial
> example), but there were some interesting articles, e.g.
>
> http://www.inconcept.com/jcm/June2000/becker.html
Thanks, I'll look into it.
Cheers,
Joe
Received on Sun Mar 25 2007 - 23:48:52 CEST