Re: Interpretation of Relations
From: paul c <toledobythesea_at_oohay.ac>
Date: Sat, 20 Jan 2007 02:20:19 GMT
Message-ID: <D%esh.707744$5R2.240230_at_pd7urf3no>
>>I'm very new to this databases game, and am not even sure I'm using the
>>terminology in the right way. I'd like some feedback as to whether I'm
>>even in the right ballpark. Most of my understanding of the terminology
>>comes from reading this group, and the definitions on Wikipedia.
>>
>>I've been wrestling with the correct interpretation of a relation. I'm
>>currently working under the assumption that a relation comprises both a
>>Type (or header) and a Body (or values).
>>
>>I'm going to start small.
>>
>>Consider modelling a situation in which there are people, and they have
>>eye colour. I'm going to define some very small domains, so that I can
>>enumerate the facts that I believe a given relation represents. I'm
>>sorry if the notation is non-standard, but here it is.
>>
>>Domain D_People = {Joe}
>>Domain D_Hair = {Red, Blond}
>>
>>Relation R_People = <<D_People>: {{Joe}}>
>>Relation R_Hair Colour = <<D_People X D_Hair>: {{Joe, Blond}}>
>>
>>(The bit in the <> is the relation header, the subsequent sets are the
>>relation body).
>>
>>So, I should interpret this to mean that "Joes hair is blond".
>>But that is not all, because the closed world assumption means that I
>>have another fact (just one, because the domains are so small). This
>>second fact is "NOT Joes hair is red".
>>
>>And, in particular, it would be wrong to state that
>>
>>R_Hair Colour: <<D_People X D_Hair>: {}>
>>
>>indicates that I don't know the colour of Joe's hair. It really means
>>
>>NOT Joes hair is Red
>>NOT Joes hair is Blond
>>
>>Is this right?
>>If so, it leads me to a question about modelling missing
>>information. (And a lot of other questions, too). If not, is there a
>>simple thing that I've missed?
Date: Sat, 20 Jan 2007 02:20:19 GMT
Message-ID: <D%esh.707744$5R2.240230_at_pd7urf3no>
JOG wrote:
> Joe Thurbon wrote: >
>>I'm very new to this databases game, and am not even sure I'm using the
>>terminology in the right way. I'd like some feedback as to whether I'm
>>even in the right ballpark. Most of my understanding of the terminology
>>comes from reading this group, and the definitions on Wikipedia.
>>
>>I've been wrestling with the correct interpretation of a relation. I'm
>>currently working under the assumption that a relation comprises both a
>>Type (or header) and a Body (or values).
>>
>>I'm going to start small.
>>
>>Consider modelling a situation in which there are people, and they have
>>eye colour. I'm going to define some very small domains, so that I can
>>enumerate the facts that I believe a given relation represents. I'm
>>sorry if the notation is non-standard, but here it is.
>>
>>Domain D_People = {Joe}
>>Domain D_Hair = {Red, Blond}
>>
>>Relation R_People = <<D_People>: {{Joe}}>
>>Relation R_Hair Colour = <<D_People X D_Hair>: {{Joe, Blond}}>
>>
>>(The bit in the <> is the relation header, the subsequent sets are the
>>relation body).
>>
>>So, I should interpret this to mean that "Joes hair is blond".
>>But that is not all, because the closed world assumption means that I
>>have another fact (just one, because the domains are so small). This
>>second fact is "NOT Joes hair is red".
>>
>>And, in particular, it would be wrong to state that
>>
>>R_Hair Colour: <<D_People X D_Hair>: {}>
>>
>>indicates that I don't know the colour of Joe's hair. It really means
>>
>>NOT Joes hair is Red
>>NOT Joes hair is Blond
>>
>>Is this right?
> > > I see nothing wrong with your logic. What you are saying via an empty > relation is that there is no proposition for which Joe has a hair > colour. In fact I think from it you can infer: Joe has no hair colour. > (which given my external knowledge --> Joe has no hair). > >
>>If so, it leads me to a question about modelling missing
>>information. (And a lot of other questions, too). If not, is there a
>>simple thing that I've missed?
> > > My current understanding is that: > > * If an attribute is Inapplicable then simply stating no proposition > containing it is sufficient for that fact to be inferred. > * If the attribute is Applicable but we do not have value for it, then > we must state this propositionally to avoid inferring it as > inapplicable under the CWA. > * Alternatively if the attribute is 'possible' (we don't know if it is > missing or inapplicable) then we must also state a proposition > reflecting this in order to avoid the inapplicability inference. > > It is worth noting that many view db-query results as coming with the > caveat "as far as I, poor naive database, know". Jim. > > ...
If the applicable attribute with no value or possible attribute with or
without a value, should an invoice be sent to the customer?
p