Re: database design method

Jan,

> Another thing I wondered about while reading it is if I can express the
> NEST and the UNNEST operations of the nested relational algebra. I seem to
> remember that Date & Darwen were heavily opposed to those operations, but
> the language would otherwise be certainly computationally incomplete. So,
> can I?

I took a look at the printed version of TTM (2ed) (which is not available on line).

On pages 137-138 there is a discussion of NEST and UNNEST (in Tutorial-D the operations are called GROUP and UNGROUP).

So for the relationtype

RELATION { S# S#, PQ RELATION { P# P#, QTY QTY } } we can get to

RELATION { S# S#, P# P#, QTY QTY } with

(assuming SPQ1 is a relvar of the first type and SPQ2 is a relvar of the second type):

SPQ1 := SPQ2 GROUP { P#, QTY } AS PQ and the opposite operation would be

SPQ2 := SPQ1 UNGROUP PQ regards,
Lauri Pietarinen Received on Sat Nov 09 2002 - 18:55:03 CET