Re: Interpretation of Relations
From: paul c <toledobythesea_at_oohay.ac>
Date: Sat, 20 Jan 2007 03:15:03 GMT
Message-ID: <XOfsh.716941$R63.248596_at_pd7urf1no>
>>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
Date: Sat, 20 Jan 2007 03:15:03 GMT
Message-ID: <XOfsh.716941$R63.248596_at_pd7urf1no>
JOG wrote:
> paul c wrote: >
>>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
> > > charging for what? His haircut? >
Sure, why not? Received on Sat Jan 20 2007 - 04:15:03 CET