Re: Understanding some issues on the denotational semantics

On Feb 18, 2011, at 17:32 , Eric Prud'hommeaux wrote:

> * Ivan Herman <ivan@w3.org> [2011-02-18 11:40+0100]
>> Alex,
>> 
>> 
>> On Feb 17, 2011, at 16:18 , Alexandre Bertails wrote:
>> 
>> [snip]
>>> 
>>> Sorry, I did not understand your comment this way as I think that the
>>> type is already correct.
>>> 
>>>> But does that specification means that for every table t, primaryKey(t) will give me a set s=primaryKey(t) where size(s)<=1? Ie, that, for every table, the primaryKey is either empty or restricted to one single column? That is the discrepancy with section 2.2 that explicitly speaks about multi-column primary keys...
>>> 
>>>> From you comment:
>>> s/the primaryKey is either empty or restricted to one single column/the primaryKey is either empty or restricted to one single CandidateKey/
>>> 
>> 
>> Sorry, right
>> 
>>> So primaryKey's type tells you that primaryKey gives you either 0 or 1
>>> CandidateKey. The definition for CandidateKey is
>>> [[
>>> CandidateKey ::= List(ColumnName)
>>> ]]
>>> 
>>> So you do have multi-columns for a Primary Key (if there is one).
>>> 
>>> Am I getting it right? I may be not introducing this one the right way.
>>> 
>> 
>> I think I get it now but... why do I need this? Why not define
>> 
>> primaryKey : Table → CandidateKey
> 
> I think this would say there is exactly one primary key per table.

Hm. Indeed, one primary key, though that primary key would consist, possibly, of several columns. Isn't this what we want? I am really confused:-(

Ivan


> 
>> or is a List necessarily non-empty, ie, 
>> 
>> CandidateKey ::= List(ColumnName)
>> 
>> means that there _is_ at least one column (ie, the no primary Key alternative would not be covered?)
> 
> I read this as saying there are 0 or more primary keys, and, including the specification "size(s) ≤ 1" gives us 0 or 1 primary keys, modelling an optional without inventing a new type.
> 
>> Ivan
>> 
>> ----
>> Ivan Herman, W3C Semantic Web Activity Lead
>> Home: http://www.w3.org/People/Ivan/
>> mobile: +31-641044153
>> PGP Key: http://www.ivan-herman.net/pgpkey.html
>> FOAF: http://www.ivan-herman.net/foaf.rdf
>> 
>> 
>> 
>> 
>> 
> 
> 
> 
> -- 
> -ericP


----
Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
PGP Key: http://www.ivan-herman.net/pgpkey.html
FOAF: http://www.ivan-herman.net/foaf.rdf