Re: teaching relational basics to people, questions

From: <compdb_at_hotmail.com>
Date: Sat, 28 Nov 2009 15:45:59 -0800 (PST)
Message-ID: <e5de46d3-3d2b-49db-8016-2c4eade56d6d_at_j19g2000yqk.googlegroups.com>



On Nov 27, 7:32 pm, com..._at_hotmail.com wrote:
> The relationship between the two "predicates" is as follows. The
> designer specifies the relvar predicate so that a user updating the
> database can observe the world and figure out whether a given tuple
> makes it true and so that a user looking at the database can find out
> what is true of the world. Next the designer figures out all the
> possible tuples that could ever turn up in a relation because of the
> way the world can be and then writes the total relvar constraint so
> the dbms can tell the user they made an error if they ever try to put
> some other tuple into the relation.

The line
> possible tuples that could ever turn up in a relation because of the
is wrong; I should have written:
"possible values of the relation that could ever turn up because of the"
That is, the designer communicates the set of. (They actually communicate all the possible database states that could ever arise, via the "total database constraint". But above I was just addressing the term "total relvar constraint".)

philip Received on Sat Nov 28 2009 - 17:45:59 CST

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