- From: Ivan Herman <ivan@w3.org>
- Date: Fri, 18 Feb 2011 18:45:51 +0100
- To: Eric Prud'hommeaux <eric@w3.org>
- Cc: Alexandre Bertails <bertails@w3.org>, Juan Sequeda <juanfederico@gmail.com>, RDB2RDF WG <public-rdb2rdf-wg@w3.org>
- Message-Id: <D38BC8A5-D0C1-4488-8470-2A8CEC3D0A09@w3.org>
I guess what screws me up all the time is that the notation always reminds me of what I was trained namely that {...} is a set, and f:A->B is a function between sets. But these are not sets, these are types, and that is what makes it difficult for me. I guess this is the difference between the mathematician (that I was trained for) and a computer scientists (that you guys are...) I will have to look at this again... tomorrow! Cheers Ivan On Feb 18, 2011, at 18:04 , Eric Prud'hommeaux wrote: > * 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 ---- 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
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Received on Friday, 18 February 2011 17:44:54 UTC