Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

Jon Heggland schrieb:
> In article <42b182de$1_at_news.fhg.de>, savinov@host.com says...

>>There is also another problem. Sometimes there are alternative (not 
>>unique) paths along which we can propagate constraints. In this case the 
>>database also cannot help but it is not its fault. The model simply does 
>>not define what path to choose (path is used in terms of COM).
>>
>>In UR it is a serious drawback bcause it is based on relational  model 
>>(its spirit and fundamental assumptions), it is actually a complement to 
>>the RM. We need to look at the data differently in order to solve the 
>>problem.

Yes, I agree. If we have an interactivity then we can solve the problem by asking questions. But I meant that it is a problem when we design our database. At this moment the designer, if he wants to explain what the data means, has to correctly specify the structure of tables. As one extreme we can create a number of tables without any relationships where each query has to contain all instructions (declarative) for building the necessary result set. Another extreme is where all ambiguities are resolved by moving the relationships (joins) into the database where they have a persistent form and are a integral part of the database and data semantics. In this case all simple constraints (imposed on one table) can be formally correctly propagated for getting the result.

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