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Re: Little question for RDM theoristes

From: Erwin <e.smout_at_myonline.be>
Date: 16 Jun 2006 02:44:12 -0700
Message-ID: <1150451052.417077.47600@y41g2000cwy.googlegroups.com>


> I just think that relation is value and is whole value. There is no
> some special operation on header - only relational algeba exist, which
> manipulates with relations (i.e. both with headers and bodies
> together). For example when we make carthesion products (or other
> rel.operation) of two rel.values we get third rel.value, schema of
> which defined by operand (and by operation). It means that we cannot
> "change" header because we cannot "change" realtion, becouse relation
> is value (whole value) and value cannot be changed.

You're saying exactly the same thing as I (think I) did. So I really cannot understand how/why you say you cannot accept what I said as an argument.

> Of course some implementation of RMD can have some operations to work
> with rel.variables(not values!) - something like ADD ATTRIBUTE (in SQL
> it looks like ALTER TABEL tablename ADD columnname). But result is
> other value then the one contained by this relvar before such operation
> on this relvar was done. I'm not sure, but in term of RMD such
> operation can mean carthesian product on previous value and empty
> unary relation.

"Empty unary relation" is wrong, I think. I think it should be "singleton unary relation". Not degree one, cardinaltiy zero, but degree one, cardinality one.

So you agree that changing the header of a certain relation variable *requires* one to do something about the current value of that variable, i.e. it *must* be assigned *another* value than the one that is currently there ?

How can one explain this to be a *requirement* if it does not derive from the fact that R1 has changed type (and that the current value simply isn't of that new type) ? Received on Fri Jun 16 2006 - 04:44:12 CDT

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