- From: Eric Prud'hommeaux <eric@w3.org>
- Date: Fri, 18 Feb 2011 12:04:10 -0500
- To: Ivan Herman <ivan@w3.org>
- Cc: Alexandre Bertails <bertails@w3.org>, Juan Sequeda <juanfederico@gmail.com>, RDB2RDF WG <public-rdb2rdf-wg@w3.org>
* Ivan Herman <ivan@w3.org> [2011-02-18 17:49+0100] > > 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:-( What we want is zero or one primary key with 1 or more columns, which I believe is captured by candidateKeys : Table → {l:List(CandidateKey) | size(l) ≥ 1} primaryKey : Table → {s:Set(CandidateKey) | size(s) ≤ 1} (I added a size constraint on CandidateKey and note that a Set of 0 or 1 is the same as a List of 0 or 1.) > 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 > > > > > -- -ericP
Received on Friday, 18 February 2011 17:04:46 UTC