Re: database design method

> >
> > So {} is a valid value for this tuple type. Excuse my ignorance, but what is
> >it? A tuple with two attributes but no 'rows'?!
>
> Look in the type definition. The type of 'subtrees' is a RELATION type. The
> {} denotes the empty set, i.e., an empty relation.

This might be beside the point but TTM
includes two standard tables, i.e.

TableDee :=
 relation {
   row {} };

and TableDum :=
 relation { };

They are the '0' and '1' elements
of the relational algebra.

• proscription #5: 5. D shall not forget that relations with no attributes are respectable and interesting, nor that candidate keys with no components are likewise respectable and interesting.

For further elaboration see Date's
selected writings (1989-1991).
There are some beautiful articles
by Darwen.

Sorry if this was a distraction.

regards,
Lauri Pietarinen Received on Tue Nov 12 2002 - 10:47:25 CET