Re: modeling either/or relationship...

David Cressey schrieb: 
"Volker Hetzer" <volker.hetzer@xxxxxxxx> wrote in message
news:drd0q3$idc$1@xxxxxxxxxxxxxxxxxxxxxxxxxxx

Hi!
I've got to model electrical constraints on a net of a pcb.
So, a net can be constrained either by layer,net class (and constraint
name) 
or constraint class (and constraint name).

The problem I have is that a constraint (with a name and a bunch of
attributes) 
can be either
- a "free constraint, adressed by layer, net class and name or
- a "bound constraint", adressed by the constraint class name.
A constraint cannot be both.

I can enforce this easily with a table constraint (id set1 xor id set2 is
true), but 
can I put this into an ER diagram too?

Lots of Greetings!
Volker

Look up "generalization specialization relational model" on the web. 
Did that before asking here. (I saw your response to a related question.)
What I saw was that triangle thingy. Can I assume that when some tools
have a "subcategory relationship" looking like "0||", that they mean the
same? If yes,

then, change of problem description:
I'm looking for a database design tool that, when doing that subcategory
relationship, creates code that enforces the constraint that an object
can belong to only one subcategory.

What I'm currently doing is to have three tables (root + 2 subs),
root has a type field and each sub has a type field too and a constraint
forcing the sub type to be of a particular value. Sub name and type
have to exist in root and there is only one row per name in root.
My current database tool (dezign) is useless with subcategories, so
I have to fiddle with two non identifying 1:n relationships instead
of a subcategory relationship.

Lots of Greetings and thanks for bothering!
Volker
.

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