Re: Missing Information & Empty Sets.

"byrmol" <member27348_at_dbforums.com> wrote in message news:3312333.1062484370_at_dbforums.com...
>
> 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?

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

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

No. Set values are values of set domains.

> 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?

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

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

Not applicable.

> 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.

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

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

Harsh? Received on Tue Sep 02 2003 - 17:09:27 CEST