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Re: database design method

From: Jan Hidders <hidders_at_REMOVE.THIS.uia.ua.ac.be>
Date: 11 Nov 2002 16:14:09 +0100
Message-ID: <3dcfc941$1@news.uia.ac.be>


Paul Vernon wrote:
>"Jan Hidders" <hidders_at_REMOVE.THIS.uia.ua.ac.be> wrote in message
>news:3dcda833$1_at_news.uia.ac.be...
>>
>> This is of course their good right but I hope the authors realize how much
>> better for the acceptance of their model it would be if it were on line.
>> Think of ODMG/OQL and W3C/XML.
>
>How much difference do you think it would make? I agree that their model
>could certainly be 'marketed' with more, err pazazz.

Indeed, and if the tone at dbdebunk became a bit less aggressive people might actually start to listen again.

>> [...] 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 have the book on me today, but I'm sure that they explicitly disallow
>such recursive definition of both tuples and relvars.
>
>Are there arguments in favour of recursively defined types in the relational
>model? I don't recall D&D explaining why they disallow them.

Trees are a very common data structure. Also think of semistructured data. If you don't support that then people will add their own domains to store them in and then they will be out of reach of the query optimizer.

>What would a valid value of Tree look like?
>
>{"NodeA:Value1", ("NodeB:Value2", ("", {}) )}

Yeah, something like that.

>How do we avoid recursing infinitely?

Usually by limiting recursion to structural recursion.

Received on Mon Nov 11 2002 - 09:14:09 CST

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