# Re: SQL, related records (quotes)

Date: Sun, 26 Jun 2005 11:38:13 GMT

Message-ID: <Fswve.2471$J12.893_at_trnddc05>

"Stefan Rybacki" <stefan.rybacki_at_gmx.net> wrote in message
news:3i78t2Fk4c3tU1_at_individual.net...

*> Dan Guntermann wrote:
**> >...
*

>> But alas, not many implementations allow for assertions of antisymmetry,

*>> though it could be done with a trigger. This approach would enforce the
**>> condition that
**>> for all (child, parent) relationships that are members of hierarchies,
**>> there does not exist a tuple of (parent, child). It also has the
**>> limiting factor of disallowing single node hierarchies.
**>
**> You meant non-symmetry since anti-symmetry says: you can have (child,
**> parent) and (parent, child) at the same time execept parent=child
*

Deesn't non-symmetry simply mean there exists a tuple <child X, parent Y> in hierarchies such that there is no corresponding tuple <child Y, parent X> in hierarchies? This isn't the same as a universal quantifier.

child parent 6 3 3 6

If it doesn't, then it makes sense that the DBMS reject such a condition. Obviously 6 does not equal 3.

*>
*

> Just mentioned ;) (I know antisymmetry works here since you said you don't

*> allow reflexive tupels)
*

Right. Suppose a relation R on the domain A. For all a, b in the same
domain A,

a R b ^ b R a --> a = b, which is equivalent to
~(a R b ^ b R a) V a=b.

However, we have a constraint that asserts the condition a <> b. Thus, ~(a
R b ^ b R a) V FALSE reduces to:

~(a R b ^ b R a).

*>
**> Regards
**> Stefan
**>
**> >...
*

Regards,

Dan Received on Sun Jun 26 2005 - 13:38:13 CEST