byrmol

I have been doing a bit of thinking about the Missing Information
problem in both SQL and TRDBM systems

and would like some feedback in relation (pun intended) to those
thoughts.

Assume a simple Customer - Order schema

Customer (Customer# INT) KEY (Customer#)

Order (Order# INT, Customer# INT) KEY (Order#) FOREIGN KEY
(Customer.Customer#)

The Customer# domain is a set of unsigned integers from 1 -> INT.MAX

Lets assume that the Customer relation has no tuples ie: is a empty set.

The domain for the Customer# attribute in the Customer relation is still
1 -> INT.MAX

Now here is where I sink or swim........

BUT the domain for the Customer# attribute in the Order relation is
effectively an empty set

due to the RI constraint. The domain "grows" as tuples are added to the
Customer table.

So from there I have a couple of questions

1) Is an empty set of type X a valid value of the domain X?

2) Does an empty set of type X violate 1NF?

So in context it boils down to this question...

Can I insert an empty set (with the relevant empty-set types) into the
Order table without violating RI?

If the answer is YES then could we not simply use an empty-set as the
Missing Information marker?

I got these thoughts from TABLE_DEE and TABLE_DUM as outlined in TTM but
unfortunately

my copy is currently 1200Km away and I am struggling to remember there
definitions correctly.

All criticism welcome however harsh.......


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Posted via http://dbforums.com

bob badour

No. It is a valid value from of the domain set of domain X.


No. Set values are values of set domains.

No. The order table does not have a set-valued or relation-valued attribute;
it has an integer-valued attribute.


Not applicable.


TABLE_DEE is a nullary relation with cardinality 1 and TABLE_DUM is a
nullary relation with cardinality 0.


Harsh?

byrmol

Thanks Bob for succint reply..

I added harsh because I see some atrocious ad hominem attacks in
this forum...

In rugby terms.. Play the Ball not the man....

In reference to question 1..

If we extend every Domain to be a set domain would this allow us to use
an empty set as the Missing Information marker?

ie.. The INT Domain now becomes this {{1}, {2}...{n}}

instead of {1,2 .....n}


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Posted via http://dbforums.com

bob badour

The INT domain would become {{},{0],{0,1},{0,2}...{n,n,n,n,n...}} if we
arbitrarily allow a set of INT where we now allow INT. It does not seem like
a sufficiently cautious proposal to me.

Relation-valued attributes already allow us to represent optional data as a
constrained set where we can enforce a maximum cardinality of 1 by
specifying a nullary key.