Lots of Idiotic Silly Braces?
Date: Mon, 16 Jul 2007 19:14:14 GMT
Message-ID: <asPmi.123158$NV3.2515_at_pd7urf2no>
No big reason for wondering about this, the uses may seem obscure but also seem interesting to me, David C and his notation thread has sent me off on what could be another wild goose chase. Gave this post a different subject so as not to usurp his.
I think that some relations, once grouped and then projected, can't be presented as a single table.
Eg.,
ShipmentParts{S,P} // (ie., Shipment number, Part number):
S P
1 3
1 4
2 3
If we group on Part, we produce a relation that has two tuples instead of the three above and I might make it graphic like so:
S {P}: 1 {3,4} 2 {3}
Parts Combination (b):
P
3
As tables they are cosmetically different, they have different numbers
of rows and one mentions Suppliers while the other mentions sets of
Suppliers but they could represent the same relation under the covers.
Most implementations seem to echo the table with two rows but it seems
to me just as reasonable to echo the one that has only one row, ie.,
{S}
and when the system is asked to present it, it appears as
S
(The reason I say that as "tables", S and {S} are only different in a
cosmetic way, is that I prefer to think that it is not tables that a
dbms operates on, rather only relations.)
But I can see no reason why there couldn't be an {S} relation that
looked like:
{S}
and no other way (in the scheme I have in mind) to present it as a table
than:
S
So, in this example, the relation "form"
{S}
{1,2},
1
2.
{1}
{2}
1
2
{1}
{2}
happens to be equal (in information) to the "form"
{S}
{1,2}.
p Received on Mon Jul 16 2007 - 21:14:14 CEST