Re: storing survey answers of different data types

Brian Selzer wrote:
...
> It doesn't mean that Codd's model is broken: it means that your
> interpretation of Date's Principle of Interchangability is just plain wrong.
> The distinctions between base and derived relvars that limit the kinds of
> updates that are permissible are neither arbitrary nor unnecessary. An
> insert into a relvar that is the union of two base relvars is inherently
> ambiguous, since it implies an insert into at least one but possibly both of
> the base relvars. Disambiguation is possible, but only through the use of
> multiple assignment--in fact, Date's Assignment Principle demands the use of
> multiple assignment. In the same way a delete from a relvar that is the
> join of two base relvars is inherently ambiguous, since it implies a delete
> from at least one but possibly both of the base relvars. Again,
> disambiguation is possible, but only through the use of multiple assignment.
>

In the face of the actual situation, that is all just chatter. Quoting Date from a few years ago at: http://www.dbdebunk.com/page/page/1396086.htm

(quote)
In other words, the operation
DELETE SSP WHERE S# = S#('S1') AND P# = P#('P1') ; is inherently unsafe, since we presumably don't know, in general, whether there are any other shipments for supplier S1. (end quote)

In the exchange, Date said the Delete operation fails, so I presume SSP remains unchanged. Instead of that delete statement, imagine this:

Select SSP where S# Not Equal S#('S1') OR P# Not Equal P#('P1') ;

I presume the Select operation doesn't fail under Date's principles. So a user could select a relation that the dbms refuses to record. If the Delete is 'unsafe', so is the Select. So sometimes the relation's tuples are true and sometimes the same relation's tuples aren't true.. Looks broken to me. Received on Sat Apr 25 2009 - 19:57:56 CEST