>> From: Patrick Cassidy
>
>> In the RDB's I have seen, each field (column in a table) often
> (typically?)
>> represents one relation on the type represented by the table, so the
> number
>> of columns does not have any relation to the arity of the relations
> used.
>> Two or more columns can be used together to represent higher-arity
>> relations, but they may be quite independent.
>
Reply from Jeff Schiffel
> I'm not certain I know what you mean. In an entity-relationship isn't
> the relation the entire row, not just one column? The entity is the
> name of the table. The names of the columns are the identifiers of an
> n-ary relation. Each row is an instance of the n-ary relation. Or am I (01)
> missing something?
> (02)
Pat Hayes adds:
Thats one way to think about it. But another way is that each column of
the table is a binary (!) relation holding between the table and the
rows, which are entities. This is the same switcheroo which takes an
n-ary relation into a collection of binary 'case roles' (think column
names) in the format beloved of linguists. Basically, its the rather
trite observation that a 2-d table can be thought of either as a vector
of rows or a vector of columns. Both ways work. (03)
Jeff again: (04)
Yes, but I tend not to think about it that way, probably due to my
information systems background. Your point, however, is very good, if I
catch your meaning. Let's say I find a relation that isn't binary by
itself between a column and the table. Then I have found a candidate for
normalization, since it is a masquerading binary that should be further
decomposed (into another table where the dependent attribute becomes
binary with the new table). (05)
-- Jeff Schiffel (06)
_________________________________________________________________
Message Archives: http://ontolog.cim3.net/forum/ontolog-forum/
Config Subscr: http://ontolog.cim3.net/mailman/listinfo/ontolog-forum/
Unsubscribe: mailto:ontolog-forum-leave@xxxxxxxxxxxxxxxx
Shared Files: http://ontolog.cim3.net/file/
Community Wiki: http://ontolog.cim3.net/wiki/
To join: http://ontolog.cim3.net/cgi-bin/wiki.pl?WikiHomePage#nid1J
To Post: mailto:ontolog-forum@xxxxxxxxxxxxxxxx (07)
|