Re: database design method

Lauri Pietarinen wrote:
>Not so fast, Jan
>
>> >On pages 137-138 there is a discussion of NEST and UNNEST
>> >(in Tutorial-D the operations are called GROUP and UNGROUP).
>>
>> Wow!! Do you realize what this means? Chris Date has recanted! The
>> relational model is dead, long live the nested relational model. We can
>> finally kiss the 1NF good bye. Good riddance, I say. I think I'm beginning
>> to like Tutorial-D. :-)
>
>To quote further from TTM (pages 136-137):
>
><quote>
>
>Note that we permit relations (and tuples) to include attributes whose
>values are relations. (However, we explicitly do not espouse "NF squared"
>relations as described in e.g. reference [121], because they
>involve major extensions to the classical relational algebra,
>extensions that we find unnecessary.)
>
>[121] Mark A. Roth, Henry F. Korth, and Abraham Silberschatz:
>"Extended Algebra and Calculus for Nested Relational Databases,"
>ACM TODS 13, No.4 (December 1988)
>
></quote>

These "major extensions" they talk about are the NEST and UNNEST operators. You have just yourself told me that these can be done in Tutorial-D. The inescapable conclusion then is that it is therefore a superset of the nested relational algebra. So it walks like a duck, it talks like duck, and it swims like a duck. Why Date and Darwen want to deny that it is is a duck is a complete mystery to me.

>What I understand of this is that the attribute is seen by the relational
>operators as just an attribute. We have to separately "open up" that
>attribute to see what's inside it and further operate on it.

Roughly that is also what happens in the nested relational model.

>> So can I abuse you a little more as my tutorial-D tutor? :-) Can I have
>> recursive types? For example (probably not the right syntax, but I think you
>> will understand what I mean):
>>
>> TYPE Tree = TUPLE { NODE-VALUE STRING, SUBTREES RELATION { TREE Tree } }
>>
>
>I don't really know if what you suggest here is permissible - perhaps
>it is. However there are no relational operators that would "traverse" the
>tree. You would have to explicitely "open up" each tree-tuple separately.
>
>What are you trying to achieve with this construct? Is it something
>that can't be done any other way? What would this be useful for?

Recursive types are used in lots of programming languages. For a concrete example think of parsing trees or XML documents. Of course you can simulate those with unnested tables and non-recursive types but then you have to introduce artificial node identifiers.

• Jan Hidders