Re: repeating groups

From: mAsterdam <mAsterdam_at_vrijdag.org>
Date: Sun, 19 Feb 2006 12:03:32 +0100
Message-ID: <43f84ffd$0$11067$e4fe514c@news.xs4all.nl>

Jonathan Leffler wrote:
> mAsterdam wrote:

>> Marshall Spight wrote:
>>
>>> ... there are several ways out of the repeating groups problem
>>>
>>> 1) decomposing relations, aka "classical" 1NF
>>> 2) higher-than-1 cardinality attributes: lists or sets
>>
>> Aren't you jumping over the "order may have meaning" problem
>> here by taking these two together?

>
>
> It depends whether you take the OR in 'lists or sets' to be inclusive or
> exclusive, doesn't it? If it is inclusive, then lists handle the 'order
> has meaning' case and sets handle the 'order is not significant case'.
> But you've complicated the algebra - instead of just sets, you've now
> got to deal with lists and sets (and the alternatives you list below).

In the products I know the choice is made for us. In the XML family every order is kept (treated as significant - even when it isn't/shouldn't be), in the SQL family in everything except in the list of characters (varchar) order is thrown away (treated as insignificant).

>>> 3) Fully nested relations.
>>
>>
>> lists-of-lists? sets-of-sets? lists-of-sets? sets-of-lists?
>>
>>> Actually, 2) was something I hadn't really thought of before:
>>> add *one* level of nesting. This idea is interesting but less
>>> appealing than 3, because it is less regular. I tend to prefer
>>> orthogonal designs; they have fewer arbitrary limitations.

>
>
> Was it Hoare who said "anything in computer science that isn't recursive
> isn't any good" or words to that effect? (I found two references at
> Google to Jim Gray saying "Anything in computer science that's not
> recursive is no good", but it is not an easy expression to search for.)

Nice.

> 1-level of nesting would be arbitrary and would very quickly become
> restrictive. Indeed, you'd have to be very careful not to lose the
> closure property of the algebra.
>
> >[...snippage...]

Time for the obligatory Einstein quote (and some more) on simplicity: http://www.quotegarden.com/simplicity.html Received on Sun Feb 19 2006 - 05:03:32 CST