Re: Bidirectional Binary Self-Joins

David Cressey wrote:

"JOG" <jog@xxxxxxxxxxxxx> wrote in message
news:1175268237.587602.313730@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

On Mar 30, 3:44 pm, "Marshall" <marshall.spi...@xxxxxxxxx> wrote:

On Mar 30, 6:40 am, "David Cressey" <cresse...@xxxxxxxxxxx> wrote:




"JOG" <j...@xxxxxxxxxxxxx> wrote in message

"There was a game on Date [Dec-12], where the Home team [Hope]

scored

[59] and the Away Team [Calvin] scored [32]"

Then a schema of ( Date, Home_team, Home_score, Away_team,

Away_score)

is sufficient. However if there was no such distinction between home
and away teams (perhaps you are recording finals played at neutral
grounds), then your proposition is more like:

"On Date [Dec-12], [Hope] played [Calvin], with [Hope] scoring [59]
and [Calvin] scoring [32]"

The conjunction in that sentence sets alarm bells off in my head,
because it means I could normalize this proposition to:
"On Date [Dec-12], [Hope] played [Calvin]"
"On Date [Dec-12], [Hope] scored [59]"
"On Date [Dec-12], [Calvin] scored [32]"

Then I realize I have two different types of propositions, and hence
two different types of relations. This corresponds to your design

for

1a, but it highlights the process of how I would have started to get
there. It also shows that you do not need some sort of artificial ID
key - the date and the team are enough to serve as identity.

Now, what is bugging me however is that my first relation would have
to have attributes team1 and team2, which is entirely

unsatisfactory.

RM prohibits the repetition of attribute names, even when the
attributes are playing identical, equal roles in the relationship. I
could solve this by adding a surrogate, to represent the game as a
whole, and then normalizing, but this seems worryingly artificial,
given in RL a game is identified by the teams and the date. Any
illumination from cdt is more than welcome.

This is bugging me, too. In particular, it seems to me that the
proposition:

"On Date [Dec-12], [Hope] played [Calvin]"

and the proposition:

"On Date [Dec-12], [Calvin] played [Hope]"

Imply each other. But the way one of them is stated doesn't make that
"obvious", at least not to me.

It doesn't seem to be a severe problem; more of an annoyance really.

Idle thought: if you had RVAs, you could do something like:

{ date=12-Dec, teamscores={(team=Hope, score=59), (team=Calvin,
score=32)}}

Marshall

I hate the idea of having to invent a surrogate key to identify a
game, just so that we can model it in RM (especially given it is
perfectly identifiable from the date and teams). Hence using RVA's
certainly seems preferable. But then we've added seemingly unnecessary
complexity to our queries compared to:
{ (date:12-dec, team: Hope, team:Calvin) }
{ (date:12-dec, team: Hope, score 59), (date:12-dec, team: Hope, score
32) }
Someone fix my thinking. quick.

got no quick fix. Sorry. I am going to point out that a game can be
identified by a date and a SET of teams, not a LIST of teams. Emphasis
mine. So the old bugaboo about sets versus lists surfaces once again.

Marshall's description, in terms of RVA's obscures the difference between a
set and a list, because when you lay it out in text, a set looks like a
list.

Marshall, for your RVA solution, how would the query language handle the
question,
"Find all the games that Hope played"?

Also, let's say you already had the game you stated in the database, and
you tried to insert the following game:

{ date=12-Dec, teamscores={(team=Calvin, score=32), (team=Hope,
score=59)}}

Would the database engine detect that the game was already in the relation?
If so, how would it do that? I realize my question is a physical one, not a
logical one, but I'm asking anyway.

It would use the equality operation to detect the duplicate. What's more disturbing is it would allow the following without complaint:

{ date=12-Dec, teamscores={(team=Calvin, score=31), (team=Hope, score=59) }}
.

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