Re: teaching relational basics to people, questions

From: <compdb_at_hotmail.com>
Date: Mon, 14 Dec 2009 11:33:33 -0800 (PST)
Message-ID: <d84d3f25-b398-4c35-b97d-9155baaefab4_at_o9g2000prg.googlegroups.com>



On Dec 4, 4:26 pm, com..._at_hotmail.com wrote:
> So please start out with what the designer gives, then tell me clearly
> how to form the proposition is that is "the information represented by
> each row" that is syntactically valid, then what the proposition is
> that is "the information content of a table".

On Dec 11, 12:07 am, com..._at_hotmail.com wrote:
> Thanks, but this is no clearer.

Mr. Scott,

I received an early xmas gift from my database designer: a relational dbms. Its database has relation variables vr{A,B,C,D} and vs{A,X} with respective predicates pr(A,B,C,D) and ps(A,X). Observing the world I can determine for a particular tuple whether a given predicate is true with corresponding attributes/parameters substituted. All attributes are of the same type.

At some point when the database reflects the world:

If pr(a,b,c,d) is true, what does this imply about the tuples in vr? If pr(a,b,c,d) is false, what does this imply about the tuples in vr? If a tuple <a,b,c,d> is in vr, what does this imply about the world, in terms of pr?
If a tuple <a,b,c,d> is not in vr, what does this imply about the world, in terms of pr?
What does vr imply about the world?
What does the database imply about the world?

What query (relation expression) returns: The set of tuples <a,b,c,d> for which pr(a,b,c,d)? The set of tuples <b,c,d> for which there exists an A such that pr (A,b,c,d)?
The set of tuples <a,b,c,d,x> for which pr(a,b,c,d) and ps(a,x)? The set of tuples <a,b,c,d> for which pr(a,b,c,d) and not ps (a',b',c',d')?

Each of the following queries returns a relation. For each query, what does the returned value imply about the world? vr
vr project all but {A}
vr join vs
vr minus relation{A a',B b',C c',D d'}

Maybe you don't consider these to be appropriate questions. If so please give precise rules mapping between the world and the database state and between queries and predicates.

philip Received on Mon Dec 14 2009 - 13:33:33 CST

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